pgsql: Remove freeBackends counter from the sinval shared memory area.

Log Message:
-----------
Remove freeBackends counter from the sinval shared memory area. We used to
use it to help enforce superuser_reserved_backends, but since 8.1 it's
just been dead weight.

Modified Files:
--------------
pgsql/src/backend/storage/ipc:
sinvaladt.c (r1.69 -> r1.70)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/sinvaladt.c?r1=1.69&r2=1.70)