This patch removes the hardcoded check for superuser privileges when
executing replication origin functions.  Instead, execution is revoked
from public, meaning that those functions can be executed by a superuser
and that access to them can be granted.

Author: Martín Marqués
Reviewed-by: Kyotaro Horiguchi, Michael Paquier, Masahiko Sawada
Discussion: https:/postgr.es/m/CAPdiE1xJMZOKQL3dgHMUrPqysZkgwzSMXETfKkHYnBAB7-0VRQ@mail.gmail.com