sound: Retire snd_mixer->busy

Does not really serve any real purpose. It gets set on mixer_open() and
unset on mixer_close(), so it essentially tells us whether the mixer is
open or not.

mixer_close() uses it to return EBADF in case the mixer is not busied,
as in, the mixer has not been open()'d yet. This is redundant. The other
place where this is used is to decide whether to serve an ioctl issued
by userland, in which case it won't if, again, the mixer has not been
busied (i.e., opened). Again, seems redundant.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53859