ASoC: soc-core: remove snd_soc_get_dai_substream()

No driver is using snd_soc_get_dai_substream(),
and snd_soc_get_pcm_runtime() is enough for such purpose.
We can revival it if it was needed in the future.
Let's remove unused function.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/87d0cxyq6k.wl-kuninori.morimoto.gx@renesas.com


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