Fix GTK+ errors during wxFilePickerCtrl destruction after recent changes.

We do need to destroy the dialog (see #15156), but we must not destroy its
GtkWidget as it is owned by GtkFileChooserButton itself, so amend the changes
of r74030 to only destroy the C++ object, not GTK+ one.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74042 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775