Retire snd_gusc ISA sound card driver

snd_gusc supported the Gravis UltraSound MAX and UltraSound PnP sound
cards.

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