Remove wxTextCtrl::OnEnabled() hack from wxGTK.

Remove wxTextCtrl::OnEnabled() hack from wxGTK.

Don't change the background colour when the control is being enabled or
disabled, it doesn't seem necessary and it's unclear why was this added by
r10179 in the first place. It does result in problems however as it could
somehow make the selection of wxTextCtrl invisible when it lost focus and so
fixes a serious usability problem which happened to all wxTextCtrls for which
a wxEVT_UPDATE_UI handler using wxUpdateUIEvent::Enable() was defined.

Closes #14898.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73756 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775