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/generic/bmpcboxg.cpp

@@ -278,8 +278,6 @@ wxSize wxBitmapComboBox::DoGetBestSize() const
 
         if ( h2 > sz.y )
             sz.y = h2;
-
-        CacheBestSize(sz);
     }
 
     return sz;