Remove obsolete SLRU wrapping and warnings from predicate.c.

Remove obsolete SLRU wrapping and warnings from predicate.c.

When SSI was developed, slru.c was limited to segment files with names in
the range 0000-FFFF.  This didn't allow enough space for predicate.c to
store every possible XID when spilling old transactions to disk, so it
would wrap around sooner and print warnings.  Since commits 638cf09e and
73c986ad increased the number of segment files slru.c could manage, that
behavior is unnecessary.  Therefore remove that code.

Also remove the macro OldSerXidSegment, which has been unused since
4cd3fb6e.

Thomas Munro, reviewed by Anastasia Lubennikova

Discussion: https://postgr.es/m/CAEepm=3XfsTSxgEbEOmxu0QDiXy0o18NUg2nC89JZcCGE+XFPA@mail.gmail.com