Delete contrib/xml2's legacy implementation of xml_is_well_formed().

Delete contrib/xml2's legacy implementation of xml_is_well_formed().

This function is unreferenced in modern usage; it was superseded in 9.1
by a core function of the same name.  It has been left in place in the C
code only so that pre-9.1 SQL definitions of the contrib/xml2 functions
would continue to work.  Six years seems like enough time for people to
have updated to the extension-style version of the xml2 module, so let's
drop this.

The key reason for not keeping it any longer is that we want to stick
an explicit PGDLLEXPORT into PG_FUNCTION_INFO_V1(), and the similarity
of name to the core function creates a conflict that compilers will
complain about.

Extracted from a larger patch for that purpose.  I'm committing this
change separately to give it more visibility in the commit logs.

While at it, remove the documentation entry that claimed that
xml_is_well_formed() is a function provided by contrib/xml2, and
instead mention the even more ancient alias xml_valid().

Laurenz Albe, doc change by me

Patch: <A737B7A37273E048B164557ADEF4A58B53962ED8@ntex2010a.host.magwien.gv.at>