tty: Retire zombie dtrwait

Nothing uses dtrwait anymore. This was elimianted with the tty mpsafe
rewrite for FreeBSD 8. Only these zombie symbols and functionality
remain. GC them. Add comcontrol to the list things to remove in 16.0.

Sponsored by: Netflix
Reviewed by: kevans, emaste
Differential Revision: https://reviews.freebsd.org/D52317