Remove the scrollbar highlighting when clicked

It does not 'de-highlight' and it was only active for the Summary view
(making it more consistent with the rest of YAFC)

Yafc.UI/ImGui/ScrollArea.cs

@@ -113,7 +113,7 @@ private void BuildScrollBar(ImGui gui, int axis, in Rect scrollbarRect, in Rect
                     }
                     break;
                 case ImGuiAction.Build:
-                    gui.DrawRectangle(scrollerRect, gui.IsMouseDown(scrollbarRect, SDL.SDL_BUTTON_LEFT) ? SchemeColor.GreyAlt : SchemeColor.Grey);
+                    gui.DrawRectangle(scrollerRect, SchemeColor.Grey);
                     break;
             }
         }