power: supply: ab8500_chargalg: Drop charging step

There is a sysfs ABI to change the "charging step" of the
charger i.e. limit how much we charge from userspace.

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>