ALSA: gus: Remove unused gus_instr.c

The instrument layer codes have been dropped years ago, but this file
was left intentionally with a slight hope for the new implementations.
But, looking at the reality, the probability we'll have a new code for
ISA GUS board is very low, and I won't bet it.  So, rather clean this
legacy stuff up.  Developers can still refer to the old code via git
history.

Reported-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Takashi Iwai <tiwai@suse.de>