ASoC: Intel: Skylake: Fix unused variable warning

With compiler option W=1, we have one more warning in the driver for
'set but unused variable', so remove the unused variable to fix it.

sound/soc/intel/skylake/skl-pcm.c: In function ‘skl_platform_open’:
sound/soc/intel/skylake/skl-pcm.c:954:26: warning: variable ‘runtime’ set but not used [-Wunused-but-set-variable]
  struct snd_pcm_runtime *runtime;

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Mark Brown <broonie@kernel.org>