Completing removal of ATM support

From: Brooks Davis <brooks_at_freebsd.org>
Date: Thu, 02 Mar 2023 23:33:46 UTC
Prior to FreeBSD 12, I removed most ATM support, but left NgATM and
ng_atmllc.  The time is to remove them is almost certainly past.  I aim
to commit deprecation notices shortly and follow up with removal after a
week or so.  If you find they are useful (in production) on systems
without support for ATM NICs, please speak up so we can discuss a
different timeline.

The proposed big removal commit for NgATM is:

https://reviews.freebsd.org/D38879

ng_atmllc is removed in

https://reviews.freebsd.org/D38880

Deprecations and a few cleanups are linked in the stack of commits.

Thanks,
Brooks