ASoC: SOF: Remove SOF_IPC_EXT_DMA_BUFFER

This enum code, and what's more important, related structures is
unused in whole source code, so it shouldn't be kept.

Signed-off-by: Karol Trzcinski <karolx.trzcinski@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200312200622.24477-4-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>