power: supply: ab8500: Drop external charger leftovers

Some leftover code for external chargers only used with
unreleased ASIC revisions and the header file for the
unsupported PM2301 was left behind in an earlier cleanup,
fix it by deleting the remnants.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>