Fix compile warnings in SDL examples

examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp

@@ -152,7 +152,6 @@ bool ImGui_ImplSdlGL3_ProcessEvent(SDL_Event* event)
         }
     case SDL_TEXTINPUT:
         {
-            ImGuiIO& io = ImGui::GetIO();
             io.AddInputCharactersUTF8(event->text.text);
             return true;
         }

examples/sdl_opengl_example/imgui_impl_sdl.cpp

@@ -131,7 +131,6 @@ bool ImGui_ImplSdl_ProcessEvent(SDL_Event* event)
         }
     case SDL_TEXTINPUT:
         {
-            ImGuiIO& io = ImGui::GetIO();
             io.AddInputCharactersUTF8(event->text.text);
             return true;
         }