sound: Retire DSP_DEFAULT_SPEED

Only a few drivers use this, but this is not really our "default" speed.
And even those drivers most likely override that value at some point
once CHANNEL_SETSPEED() has been called.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53562