Don't call CacheBestSize() from DoGetBestSize() implementations

This is unnecessary, wxWindow::GetBestSize() already does this, so calling it
from DoGetBestSize() called by it too is just useless.

src/gtk/button.cpp

@@ -320,7 +320,6 @@ wxSize wxButton::DoGetBestSize() const
             ret.y = defaultSize.y;
     }
 
-    CacheBestSize(ret);
     return ret;
 }