Don't document wxFileType::GetIcon() as unimplemented under OS X.

It is implemented and supposed to work.

See #4562.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77015 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775