Remove wxMBConv_mac and wxMBConv_macUTF8D converters and use wxMBConv_cf(wxFONTENCODI...

Remove wxMBConv_mac and wxMBConv_macUTF8D converters and use wxMBConv_cf(wxFONTENCODING_UTF8) for wxConvFileName on Darwin platform regardless of GUI toolkit.
Check for Mac encodings in wxMBConv_wxwin::Init and fail to provide conversion for them via wxEncodingConverter because that code is horribly broken for several encodings.
Remove definition of wxConvLibc for non-Darwin Mac platforms (we don't support these anymore).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@47225 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775