Remove HP/Intel Itanium support.

Remove HP/Intel Itanium support.

This CPU architecture has been discontinued.  We already removed HP-UX
support, we never supported Windows/Itanium, and the open source
operating systems that a vintage hardware owner might hope to run have
all either ended Itanium support or never fully released support (NetBSD
may eventually).  The extra code we carry for this rare ISA is now
untested.  It seems like a good time to remove it.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/1415825.1656893299%40sss.pgh.pa.us