rbd-mirror: remove bogus completed_non_primary_snapshots_exist check

rbd-mirror: remove bogus completed_non_primary_snapshots_exist check

This check was added in commit ecd3778a6f9a ("rbd-mirror: ensure that
the last non-primary snapshot cannot be pruned") as an additional
safeguard against pruning an incomplete non-primary snapshot in case
there is no predecessor mirror snapshot.  However it still fires if the
predecessor is there but happens to be a primary demotion snapshot.
A bogus "incomplete local non-primary snapshot" error is reported and
the replayer gets stuck.

Remove completed_non_primary_snapshots_exist tracking as the presence
of the predecessor in the incomplete non-primary snapshot pruning arm
is already ensured by "m_local_snap_id_start > 0" condition.

Fixes: https://tracker.ceph.com/issues/56516
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>