Removed 'bool border' legacy versions of BeginChild() as they seemingly have no value other than confusing user and IDE.

Amend 7713c29 (was for #1666, #1496, #1395, #1710)

docs/CHANGELOG.txt

@@ -51,7 +51,7 @@ Breaking changes:
       After:   BeginChild("Name", size, ImGuiChildFlags_Border)
       Before:  BeginChild("Name", size, false)
       After:   BeginChild("Name", size) or BeginChild("Name", 0) or BeginChild("Name", size, ImGuiChildFlags_None)
-   Kept inline redirection function (will obsolete later) so existing code will work.
+   Existing code will still work as 'ImGuiChildFlags_Border == true', but you are encouraged to upgrade.
  - BeginChild(): Added child-flag ImGuiChildFlags_AlwaysUseWindowPadding as a replacement for
    the window-flag ImGuiWindowFlags_AlwaysUseWindowPadding: the feature only ever made sense
    for use with BeginChild() anyhow, passing it to Begin() had no effect. Now that we accept

imgui.h

@@ -337,6 +337,11 @@ namespace ImGui
 
     // Child Windows
     // - Use child windows to begin into a self-contained independent scrolling/clipping regions within a host window. Child windows can embed their own child.
+    // - Before 1.90 (November 2023), the "ImGuiChildFlags child_flags = 0" parameter was "bool border = false".
+    //   This API is backward compatible with old code, as we guarantee that ImGuiChildFlags_Border == true.
+    //   Consider updating your old call sites:
+    //      BeginChild("Name", size, false)   -> Begin("Name", size, 0); or Begin("Name", size, ImGuiChildFlags_None);
+    //      BeginChild("Name", size, true)    -> Begin("Name", size, ImGuiChildFlags_Border);
     // - Manual sizing (each axis can use a different setting e.g. ImVec2(0.0f, 400.0f)):
     //     == 0.0f: use remaining parent window size for this axis.
     //      > 0.0f: use specified size for this axis.
@@ -1018,7 +1023,7 @@ enum ImGuiWindowFlags_
 };
 
 // Flags for ImGui::BeginChild()
-// (Legacy: bit 0 must always correspond to ImGuiChildFlags_Border to be backward compatible with old API using 'bool border'.
+// (Legacy: bot 0 must always correspond to ImGuiChildFlags_Border to be backward compatible with old API using 'bool border = false'.
 // About using AutoResizeX/AutoResizeY flags:
 // - May be combined with SetNextWindowSizeConstraints() to set a min/max size for each axis (see "Demo->Child->Auto-resize with Constraints").
 // - Size measurement for a given axis is only performed when the child window is within visible boundaries, or is just appearing.
@@ -1029,7 +1034,7 @@ enum ImGuiWindowFlags_
 enum ImGuiChildFlags_
 {
     ImGuiChildFlags_None                    = 0,
-    ImGuiChildFlags_Border                  = 1 << 0,   // Show an outer border and enable WindowPadding. (Important: this is always == 1 for legacy reason)
+    ImGuiChildFlags_Border                  = 1 << 0,   // Show an outer border and enable WindowPadding. (Important: this is always == 1 == true for legacy reason)
     ImGuiChildFlags_AlwaysUseWindowPadding  = 1 << 1,   // Pad with style.WindowPadding even if no border are drawn (no padding by default for non-bordered child windows because it makes more sense)
     ImGuiChildFlags_ResizeX                 = 1 << 2,   // Allow resize from right border (layout direction). Enable .ini saving (unless ImGuiWindowFlags_NoSavedSettings passed to window flags)
     ImGuiChildFlags_ResizeY                 = 1 << 3,   // Allow resize from bottom border (layout direction). "
@@ -3150,8 +3155,8 @@ namespace ImGui
     // OBSOLETED in 1.90.0 (from September 2023)
     static inline bool  BeginChildFrame(ImGuiID id, const ImVec2& size, ImGuiWindowFlags window_flags = 0)                 { return BeginChild(id, size, ImGuiChildFlags_FrameStyle, window_flags); }
     static inline void  EndChildFrame()                                                                                    { EndChild(); }
-    static inline bool  BeginChild(const char* str_id, const ImVec2& size_arg, bool border, ImGuiWindowFlags window_flags) { return BeginChild(str_id, size_arg, border ? ImGuiChildFlags_Border : ImGuiChildFlags_None, window_flags); }
-    static inline bool  BeginChild(ImGuiID id, const ImVec2& size_arg, bool border, ImGuiWindowFlags window_flags)         { return BeginChild(id, size_arg, border ? ImGuiChildFlags_Border : ImGuiChildFlags_None, window_flags);     }
+    //static inline bool BeginChild(const char* str_id, const ImVec2& size_arg, bool border, ImGuiWindowFlags window_flags){ return BeginChild(str_id, size_arg, border ? ImGuiChildFlags_Border : ImGuiChildFlags_None, window_flags); } // Unnecessary as true == ImGuiChildFlags_Border
+    //static inline bool BeginChild(ImGuiID id, const ImVec2& size_arg, bool border, ImGuiWindowFlags window_flags)        { return BeginChild(id, size_arg, border ? ImGuiChildFlags_Border : ImGuiChildFlags_None, window_flags);     } // Unnecessary as true == ImGuiChildFlags_Border
     static inline void  ShowStackToolWindow(bool* p_open = NULL)                            { ShowIDStackToolWindow(p_open); }
     IMGUI_API bool      ListBox(const char* label, int* current_item, bool (*old_callback)(void* user_data, int idx, const char** out_text), void* user_data, int items_count, int height_in_items = -1);
     IMGUI_API bool      Combo(const char* label, int* current_item, bool (*old_callback)(void* user_data, int idx, const char** out_text), void* user_data, int items_count, int popup_max_height_in_items = -1);