mon: OSDMonitor: don't remove unamanaged snaps from not-unmanaged pools

Although we should allow creating unmanaged snaps on not-unamanaged pools,
as long as those pools don't have any managed snapshots in them, we cannot
allow removal -- because the pool will not have any unmanaged snapshots.

Fixes: 7210

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
qa: workunits/mon/rbd_snaps_ops.sh: ENOTSUP on snap rm from copied pool

'rados cppool' copies the contents but that doesn't make the destination
pool an unmanaged snaps pool.  Therefore, we must get an ENOTSUP when
we try to remove an unmanaged snap from a not-unmanaged pool.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>