ALSA: pci: Drop superfluous snd_pcm_sgbuf_ops_page

snd_pcm_sgbuf_ops_page is no longer needed to be set explicitly to PCM
page ops since the recent change in the PCM core (*).  Leaving it NULL
should work as long as the preallocation has been done properly.

This patch drops the redundant lines.

(*) 7e8edae3: ALSA: pcm: Handle special page mapping in the
                  default mmap handler

Link: https://lore.kernel.org/r/20191105151856.10785-19-tiwai@suse.de


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