ALSA: compress: Drop unused functions

snd_compress_register() and snd_compress_deregister() API functions
have been never used by in-tree drivers.
Let's clean up the dead code.

Acked-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20210714162424.4412-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>