Retire snd_ad1816 ISA sound card driver

snd_ad1816 supported ISA sound cards based on the Analog Devices
AD1816A "SoundPort® Controller".

ISA sound card drivers are deprecated as discussed on the current[1] and
stable[2] mailing lists. Deprecation notices were added in e39ec8933be4
and MFCd to stable branches.

Driver removals are being committed individually so that specific
drivers can be restored if necessary (either in FreeBSD or by downstream
projects).

[1] https://lists.freebsd.org/archives/freebsd-current/2022-March/001680.html
[2] https://lists.freebsd.org/archives/freebsd-stable/2022-March/000585.html

Reviewed by: mav
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34671