remove contrib/xml2

There has been some more discussion lately of problems caused by contrib/xml2.

http://archives.postgresql.org/pgsql-bugs/2010-01/msg00251.php
http://archives.postgresql.org/pgsql-bugs/2010-01/msg00198.php

I think we need to either (1) fix the bugs and update the
documentation to remove the statement that this will be removed or (2)
actually remove it.  Nobody seems interested in #1, so PFA a patch to
do #2.  It also rips out all the libxslt stuff, which seems to exist
only for the purpose of supporting contrib/xml2.  Per discussion on IM
with Magnus, it appears that libxslt is actually not needed to build
the backend on Windows, despite a comment in the Windows build stuff
to the contrary.

Thoughts?

...Robert