Log Message:
Drop redundant variable setting from postgresql*-server.

PGUSER, PGGROUP, and PGHOME are all in mk/defaults/mk.conf.  They are
also set (?=) in each server Makefile.  Worse, PGHOME is set to
varying values.  However, the versions in defaults prevail.
Therefore, remove the extra/confusing settings, leaving this as a
user-settable variable with a default.  pkg_info -B before and after
shows no related changes, so no PKGREVISION++.

Pointed out by Richard Palo, and ok adam@.

There's a related issue lurking, which is that PGHOME ends up being
/usr/pkg/pgsql, which is not under VARBASE, but I'm letting that be
because a change would be disruptive.