[ALSA] Remove sequencer instrument layer

Remove sequencer instrument layer from the tree.
This mechanism hasn't been used much with the actual devices.  The only
reasonable user was OPL3 loader, and now it was rewritten to use hwdep
instead.  So, let's remove the rest of rotten codes.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>