Disconnect "hide" menu signal to fix menu destruction in wxGTK.

The "hide" signal handler was triggered when destroying a sub-menu (even if it
was not shown at this time). Disconnect it to avoid asserts due to attempts to
generate an event for an already detached menu and to avoid the (bogus)
wxEVT_MENU_CLOSE event as well.

Closes #12668.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66082 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775