ASoC: soc-utils: remove dummy Platform

ALSA SoC used 2 type of Platform if sound card doesn't need
Platform.

	1) use Dummy Platform as Platform component
	2) use CPU component  as Platform component

Now, ALSA SoC allows "no Platform" settings, and it will behave
same as 2) case selection. And, all sound card which doesn't need
specific Platform are now not selecting Platform any more.

This means, no sound card is using dummy Platform on ALSA SoC any
more. This patch removes unused dummy Platform.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>