ASoC: core: Remove snd_soc_put_volsw_2r definition

We do not have users for snd_soc_put_volsw_2r anymore.
It can be removed.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>