Remove xlint(1).

xlint is currently a fossil. We have much more useful and alive tools
to do now what xlint did twenty years ago.

I did not cleared some stuff which makes lint operational, in
sys/x86/include and sys/sys, but I might do it as followup. The
x86/include/ucontext.h and _types.h hacks made to please lint was the
main reason for my initial proposal to classify xlint as obsolete and
to remove it.

Also I do not intend to clear sccs ids.

Reviewed by: bapt, brooks, emaste, jhb, pfg
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D13015