squid:osd/scrub: disable scrub reservation queuing

squid:osd/scrub: disable scrub reservation queuing

as a temporary measure for Squid RC0. There are known problems in the
existing code, with patch PRs being tested. This change disables the
Reserver functionality, until such time as the patches are merged.

Note the following trackers:
- https://tracker.ceph.com/issues/65044
- https://tracker.ceph.com/issues/65072

Once these are solved, this commit should be reverted.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>