ASoC: remove .get_regmap

To setup regmap, ALSA SoC has snd_soc_component_init_regmap() and
.get_regmap. But these are duplicated feature.
Now, no one is using .get_regmap, let's remove it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>