Don't use generic wxListCtrl style for wxListMainWindow.

Don't use generic wxListCtrl style for wxListMainWindow.

This resulted in double border around wxListCtrl contents in wxOSX and almost
certainly other ports except wxGTK for which we already had an explicit
preprocessor check in the generic list control code. Remove it and simply
don't use any wxListCtrl-specific styles, including borders, for its main
window as it doesn't make any sense.

Closes #3701.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71724 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775