remove obsolete check for GTK 2.6

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74620 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
remove ancient defines left over from GTK1
and some minor cleanup

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74621 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775