regulator: ab8500-ext: Remove unused REGULATOR_AB8500_EXT guard

Before the AB8500 External Regulator driver was Mainlined, it used
to be conditionally compiled in using the CONFIG_REGULATOR_AB8500_EXT
flag. During the review process that capability was removed, but the
guard controlling prototyping slipped though the net. This patch
cleans it up.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>