[ALSA] remove SND_GUS_SYNTH

After the removal of the sequencer instrument layer SND_GUS_SYNTH was
no longer used.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>