regulator: ab3100: Remove ab3100_regulators_remove function

Current code is using devm_regulator_register() so it is not necessary
to save reg->rdev for clean up. Remove *rdev from struct ab3100_regulator,
then ab3100_regulators_remove() can be removed as well.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>