power: supply: ab8500_chargalg: Drop enable/disable sysfs

There is a sysfs ABI to enable/disable charging of different
types (AC/USB).

Since we don't have any userspace for this code, this sits
unused and it is not used on production products either.

Drop this code.

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