ALSA: memalloc: Remove a stale comment

The comment about the reused vmalloc helpers is no longer valid after
the recent change for the noncontig allocator.  Drop the stale
comment.

Fixes: ad4f93ca ("ALSA: memalloc: Use proper SG helpers for noncontig allocations")
Link: https://lore.kernel.org/r/20211110063100.21359-1-tiwai@suse.de


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