Remove assert.h and commented out _DIAGASSERT.

Remove assert.h and _DIAGASSERT to create a paper-trail of changes
from NetBSD. Specifically didn't fix other style issues since I
don't want this to diverge from the NetBSD original too much and
that's too niggling a change to be worth future merge hassles.

Differential Review: https://reviews.freebsd.org/D16649