power: supply: axp288_charger: Remove charger-enabled state tracking

The extcon code is the only one to trigger our worker (outside of the
initial run) and we can rely on it to only call us if things have
changed, so there is no need to track the charger-enabled state.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>