Revert "Fix wxSYS_COLOUR_BTNFACE value under macOS < 10.14"

This reverts commit 8e26b8d54e because it
has been superseded by 17239d5c3d (macOS add fallback for retrieving
color channels from dynamic system colors on pre 10.14 systems,
2021-01-26) in master.

See #18977.

src/osx/cocoa/settings.mm

@@ -116,7 +116,7 @@ wxColour wxSystemSettingsNative::GetColour(wxSystemColour index)
         if ( WX_IS_MACOS_AVAILABLE(10, 14 ) )
             sysColor = [NSColor windowBackgroundColor];
         else
-            return wxColour(wxMacCreateCGColorFromHITheme(3 /* kThemeBrushDialogBackgroundActive */));
+            sysColor = [NSColor controlColor];
         break;
     case wxSYS_COLOUR_LISTBOX:
         sysColor = [NSColor controlBackgroundColor];