ALSA: isa/gus: remove snd_gf1_lfo_xxx()

There is no snd_gf1_lfo_xxx() implementation, and no one is using it.
Let's remove it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://patch.msgid.link/87o6w2pqm8.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>