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>