The shadow TLB pointer table is allocated and set up, but never really
used any longer by the driver.  Let's drop it.

Since this is the only user of snd_pcm_sgbuf_get_ptr(), we can clean
up the API after this change.

Link: https://lore.kernel.org/r/20210609162551.7842-2-tiwai@suse.deSigned-off-by: NTakashi Iwai <tiwai@suse.de>