ASoC: Remove snd_soc_cache_sync() implementation

This function has no more non regmap user, which means we can remove the
implementation of the function and associated functions and structure
fields.

For convenience we keep a static inline version of the function that
forwards calls to regcache_sync() unconditionally.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>