MFC r310547:

Remove a KASSERT which is not always true.

In case of the empty queue tp->snd_holes and tcp_sackhole_insert()
failing due to memory shortage, tp->snd_holes will be empty.
This problem was hit when stress tests where performed by pho.

PR: 215513
Reported by: pho
Tested by: pho
Sponsored by: Netflix, Inc.