zfsctl_ops_snapshot: remove methods should never be called

We pretend that snapshots mounted under .zfs are part of the original
filesystem and we try very hard to hide vnodes on top of which the snapshots
are mounted. Given that I believe that the removed operations should
never be called. They might have been called previously because
of issues fixed in r299906, r299908 and r299913.

MFC after: 5 weeks