Deprecating ISA sound cards

ISA sound cards have been obsolete for more than a decade, and it is
(past) time to retire their drivers. This includes the following
drivers/devices:

snd_ad1816  Analog Devices AD1816 SoundPort
snd_ess     Ensoniq ESS
snd_gusc    Gravis UltraSound
snd_mss     Microsoft Sound System
snd_sbc     Creative Sound Blaster

I have a review open to add deprecation notices:
https://reviews.freebsd.org/D34604

I expect to commit this in the near future, then MFC to stable
branches and remove these drivers from main. Please follow up if
there's a reason we should postpone the removal of any of these
drivers.