power: supply: generic-adc-battery: drop charge now support

Drop CHARGE_NOW support, which requires a platform specific
calculation method.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>