hier(7): drop list of /usr/include subdirectories

It is nice to have, however, the location of this information means that
it will naturally be missed by developers adding or removing directories
to the layout, so it trends out-of-date and it is out-of-date.

The target audience for hier(7) is users and administrators. It is not
expected to be a place that programmers should go to learn about the
purposes of the different C headers provided by FreeBSD.

Program authors needing FreeBSD-specific interfaces or libraries
(#include <sys/queue.h>, for instance) will either be following a more
detailed man page, or consulting the header contents directly. Folks
targeting standardized headers (#include <sys/time.h>) will not need
hier(7) to tell them where those headers are under /usr/include.

In other words, this is more detail than necessary for this document.
I'd go as far as to say that many of the existing entries in this list
do little more than parrot the name of the directory.

With all this in mind, let's drop the maintenance burden.

Reviewed by: imp, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40000