callout: retire callout_async_drain()

This function was used only in TCP before 446ccdd08e2a. It was born in
pain in 2016 to plug different complex panics in TCP timers. It wasn't
warmly accepted in phabricator by all of the reviewers and my recollection
of overall agreement was that "if you need this KPI, then you'd better fix
your code to not need it". However, the function served its duty well all
the way to FreeBSD 14. But now that TCP doesn't need it anymore, let's
retire it to reduce complexity of callout code and also to avoid its
further use.

Reviewed by: jhb, markj, kib, rrs
Differential Revision: https://reviews.freebsd.org/D43546