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>

sound/soc/soc-pcm.c

@@ -27,8 +27,6 @@
 #include <sound/soc.h>
 #include <sound/initval.h>
 
-static DEFINE_MUTEX(pcm_mutex);
-
 static int soc_pcm_apply_symmetry(struct snd_pcm_substream *substream)
 {
 	struct snd_soc_pcm_runtime *rtd = substream->private_data;