ALSA: core: Drop snd_sgbuf_get_ptr()

snd_sgbuf_get_ptr() and its sibling snd_pcm_sgbuf_get_ptr() are no
longer used by any drivers.  Let's drop them.

Link: https://lore.kernel.org/r/20210609162551.7842-3-tiwai@suse.de


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