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This function is unreferenced in modern usage; it was superseded in 9.1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.  Eleven years seems like enough time for people
to have updated to the extension-style version of the xml2 module, so
let's drop this.

We did this once before, in 20540710e, and then reverted it because
the intended change of PGDLLEXPORT markings didn't happen.  This
time the reason is to suppress link-time duplicate-symbol warnings
on AIX.  That's not worth a lot perhaps, but the value of keeping
this function has surely dropped to about zero by now.

Discussion: https://postgr.es/m/2717731.1644778752@sss.pgh.pa.us