mail/milter-manager: Fix runtime error with Ruby 3.0 or later

milter-manager requires rexml to run. Since Ruby 3.0 rexml is
converted to bundled gem. But our Ruby ports don't include bundled
gems. So fix runtime error with Ruby 3.0 or later by adding rexml port
to RUN_DEPENDS.

While here, change post-patch target so it doesn't remove bundled
glib2 gem. Build succeeds without doing it.

Reference: https://github.com/milter-manager/milter-manager/discussions/168
Reported by: Aoba Dai (in freebsd-users-jp ML)
Tested by: Aoba Dai
MFH: 2022Q3