ASoC: Remove unused 'saved_value' field from snd_soc_dapm_widget struct

The only user was removed over two years ago in commit a6c65736 ("ASoC: Remove
current PGA control handling").

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>