Don't hide the window in its dtor in wxGTK.

This results in sending an unexpected wxShowEvent to an already half-destroyed
(because the derived class dtor had been already executed) by now object which
could provoke crashes. And it also seems to be completely unnecessary as the
window is going to be destroyed anyhow -- and the other ports don't hide it
before this happens (unless it's a TLW in which case it is already hidden in
wxTopLevelWindowBase::Destroy()).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74655 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775