Tables: removed misleading, seemingy useless use of ImGuiButtonFlags_AllowtemOverlap in TableUpdateBorders(). (#6512, #3909)

This was copied from SplitterBehavior(). The only hypothetical value in SplitterBehavior() would be ability to manually call SetItemAllowOverlap() after the call.
Btw generally AllowOverlap is undesirable for columns as e.g. a spanning selectable would cover entire width and prevent columns from being used.

# Conflicts:
#	imgui_tables.cpp

imgui_tables.cpp

@@ -1208,7 +1208,7 @@ void ImGui::TableUpdateBorders(ImGuiTable* table)
         //GetForegroundDrawList()->AddRect(hit_rect.Min, hit_rect.Max, IM_COL32(255, 0, 0, 100));
 
         bool hovered = false, held = false;
-        bool pressed = ButtonBehavior(hit_rect, column_id, &hovered, &held, ImGuiButtonFlags_FlattenChildren | ImGuiButtonFlags_AllowItemOverlap | ImGuiButtonFlags_PressedOnClick | ImGuiButtonFlags_PressedOnDoubleClick | ImGuiButtonFlags_NoNavFocus);
+        bool pressed = ButtonBehavior(hit_rect, column_id, &hovered, &held, ImGuiButtonFlags_FlattenChildren | ImGuiButtonFlags_PressedOnClick | ImGuiButtonFlags_PressedOnDoubleClick | ImGuiButtonFlags_NoNavFocus);
         if (pressed && IsMouseDoubleClicked(0))
         {
             TableSetColumnWidthAutoSingle(table, column_n);