Remove pg_wait_for_backend_termination().

Remove pg_wait_for_backend_termination().

It was unable to wait on a backend that had already left the procarray.
Users tolerant of that limitation can poll pg_stat_activity.  Other
users can employ the "timeout" argument of pg_terminate_backend().

Reviewed by Bharath Rupireddy.

Discussion: https://postgr.es/m/20210605013236.GA208701@rfd.leadboat.com