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/gtk1/combobox.cpp

@@ -895,7 +895,6 @@ wxSize wxComboBox::DoGetBestSize() const
     if ( ret.x < 100 )
         ret.x = 100;
 
-    CacheBestSize(ret);
     return ret;
 }