vm_page_unswappable: remove wrong assertion

markj says:

...the assertion is incorrect and should simply be removed.
It has been racy since we removed the use of the page hash
lock to synchronize wiring of pages.

PR: 267621
Reviewed by: markj, Anton Rang <rang@acm.org>
MFC after: 1 week
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D37320