Allow GRANT on pg_log_backend_memory_contexts().

Allow GRANT on pg_log_backend_memory_contexts().

Remove superuser check, allowing any user granted permissions on
pg_log_backend_memory_contexts() to log the memory contexts of any
backend.

Note that this could allow a privileged non-superuser to log the
memory contexts of a superuser backend, but as discussed, that does
not seem to be a problem.

Reviewed-by: Nathan Bossart, Bharath Rupireddy, Michael Paquier, Kyotaro Horiguchi, Andres Freund
Discussion: https://postgr.es/m/e5cf6684d17c8d1ef4904ae248605ccd6da03e72.camel@j-davis.com