Build fix for wxOSX after the changes of r78230.

Fix wrong signature of DoHandleMenuEvent() by getting rid of this function
entirely, it's not just a trivial wrapper for ProcessMenuEvent() anyhow, so
just use the latter directly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78249 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775