ASoC: soc-pcm: remove soc_fill_silence()

No ALSA SoC driver has .fill_silence at component->driver->ops.
We can revive it if some-driver want to use it, but let's remove it
so far to avoid maintaining complex code

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87k1c55rof.wl-kuninori.morimoto.gx@renesas.com


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