regmap: Remove indexed cache type

There should be no situation where it offers any advantage over rbtree
and there are no current users so remove the code for simplicity.

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

include/linux/regmap.h

@@ -23,7 +23,6 @@ struct spi_device;
 /* An enum of all the supported cache types */
 enum regcache_type {
 	REGCACHE_NONE,
-	REGCACHE_INDEXED,
 	REGCACHE_RBTREE,
 	REGCACHE_COMPRESSED
 };