pg_upgrade: remove reference to autovacuum_multixact_freeze_max_age
autovacuum_multixact_freeze_max_age was added as a pg_ctl start
parameter in 9.3.X to prevent autovacuum from running. However, only
some 9.3.X releases have autovacuum_multixact_freeze_max_age as it was
added in a minor PG 9.3 release. It also isn't needed because -b turns
off autovacuum in 9.1+.
Without this fix, trying to upgrade from an early 9.3 release to 9.4
would fail.