followup to r329556, completely remove the covered vnode assert

vrele() acquires the vnode lock only if the hold count drops to zero.
In other scenarios it needs only the interlock. So,
zfsctl_snapdir_lookup() can race with vfs_mount_destroy() -> vrele()
such that the lookup adds a new reference and then vrele() drops the
mountpoint's reference and only then we check the reference count.
It would be just one in this case.

In fact, the assert should have been removed in r323483 when the code
learned how to deal with the uncovered vnode.

PR: 225795
MFC after: 4 days
X-MFC with: r329556