ASoC: soc-pcm: Remove unused global mutex

Since commit b8c0dab9
"ASoC: core - PCM mutex per rtd",
the global pcm_mutex is not being used any more.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>