ALSA: ice1724: remove unused array

This fixes a warning seen with clang:

sound/pci/ice1712/prodigy_hifi.c:321:28: warning: variable 'wm_vol' is not
      needed and will not be emitted [-Wunneeded-internal-declaration]
static const unsigned char wm_vol[256] = {
                           ^

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Takashi Iwai <tiwai@suse.de>