IPv4: __vax__

[Maxime Villard <max%m00nbsd.net@localhost>]

A bizarre ifndef I noticed some time ago in IPv4:

[sys/netinet/ip_output.c]
1580 #ifndef	__vax__
1581 	if (cnt % sizeof(int32_t))
1582 		return EINVAL;
1583 #endif

I don't understand why vax shouldn't have this check. The code dates back to
rev1, 25 years ago. I think we can keep the check but remove the ifndef.