ASoC: wm_adsp: Remove unused control callback structure

This callback structure has never been used and it is not clear why it
was added in the first place. Remove it to clear up the code a little.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20210211172106.16258-1-ckeepax@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>