ASoC: wm8991: remove defined but not used 'wm8991_dapm_rxvoice_controls'

ASoC: wm8991: remove defined but not used 'wm8991_dapm_rxvoice_controls'

Fix the following gcc warning:

sound/soc/codecs/wm8991.c:480:38: warning:
‘wm8991_dapm_rxvoice_controls’ defined but not used
[-Wunused-const-variable=]
 static const struct snd_kcontrol_new wm8991_dapm_rxvoice_controls[] = {
                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200407082932.41511-3-yanaijie@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>