ASoC: Remove unused snd_soc_info_volsw_ext()

The SOC_SINGLE_EXT control has been using snd_soc_info_volsw() for its info
callback since commit 1c433fbd ("[ALSA] soc - 0.13 ASoC headers"). The
snd_soc_info_volsw_ext() function has been unused ever since then, so remove it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>