Don't include pbt.h from wxMSW code unnecessarily.

The contents of this header is available from winuser.h (included by
windows.h) since at least 15 years and pbt.h itself is not included in the
latest Platform SDK releases (8+) so simply don't include it and also don't
test for its availability in configure.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69863 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
rm files (we introduced) made obsolete by wxcmake