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

lib/libc/gen/timespec_get.c

@@ -33,7 +33,6 @@
 __RCSID("$NetBSD: timespec_get.c,v 1.2 2016/10/04 12:48:15 christos Exp $");
 __FBSDID("$FreeBSD$");
 
-#include <assert.h>
 #include <time.h>
 
 /* ISO/IEC 9899:201x 7.27.2.5 The timespec_get function */
@@ -42,8 +41,6 @@ int
 timespec_get(struct timespec *ts, int base)
 {
 
-/*	_DIAGASSERT(ts != NULL); */
-
 	switch (base) {
 	case TIME_UTC:
 		if (clock_gettime(CLOCK_REALTIME, ts) == -1)