ALSA: gus: remove unused local flag

ALSA driver series for devices of Gravis Ultra Sound includes local
variable 'snd_gf1_pcm_use_dma'. Although this is a flag to change
behaviours of local implementations for 'struct snd_pcm_ops.copy' and
'struct snd_pcm_ops.silence', it's invariable during module lifetime.

This commit removes this local variable and the relevant operations.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>