ASoC: dmaengine: Remove unused SND_DMAENGINE_PCM_FLAG_CUSTOM_CHANNEL_NAME flag

There is now no users of this flag so remove it together with
related code.  The chan_name field of snd_dmaengine_dai_dma_data
data structure is not removed as it is still in use by the PXA
platform.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>