ASoC: Intel: Skylake: Remove BE prepare ops

Remove BE prepare ops which enables MCLK by default. If MCLK is required
to be enabled for any specific platform, it needs to be enabled in the
corresponding machine driver.

Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>