ASoC: Remove unused and about to be broken SND_SOC_CUSTOM I/O bus

This will be removed in -next so let's drop it from mainline as soon as
we can in order to minimise surprises.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

include/sound/soc.h

@@ -248,8 +248,7 @@ typedef int (*hw_write_t)(void *,const char* ,int);
 extern struct snd_ac97_bus_ops soc_ac97_ops;
 
 enum snd_soc_control_type {
-	SND_SOC_CUSTOM = 1,
-	SND_SOC_I2C,
+	SND_SOC_I2C = 1,
 	SND_SOC_SPI,
 };