vm_fault: Revert commit 64087fd7f372

The underlying issue that originally triggered a kernel panic was
addressed and the fix was ported to all relevant pmaps, so the
safeguards placed in vm_fault.c can be removed now.

Reviewed by: alc, kib, markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D42517