Get rid of wxMenu::m_startRadioGroup in wxOSX.

Get rid of wxMenu::m_startRadioGroup in wxOSX.

This code was probably copied from wxUniv but was wrong as we can't rely on
the items being always inserted in order. This commit on its own fixes
removing the first radio group menu item but it also makes possible to
properly implement the insertion of new items in the middle of an existing
radio group which couldn't be done with m_startRadioGroup approach at all.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74544 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775