Revert "mon: OSDMonitor: Refuse to delete CephFS pools"

This reverts commit c7d265a9b276aad5caf9b35f3ce8bc7bdd169f0f.

Because this pre-dates the `fs rm` command, this change was
preventing firefly users from ever deleting their filesystem pools.

Fixes: #8010

Signed-off-by: John Spray <john.spray@redhat.com>