ASoC: intel - kconfig: remove SND_SOC_INTEL_SST prompt

The SND_SOC_INTEL_SST is for common IPC lib and this should ideally be
not selectable symbol but selected by respective machine driver So
remove the prompt and get respective machines select it

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>