power: supply: axp288-charger: Remove unnecessary is_present and is_online helpers

Now that axp288_charger_usb_update_property() reads and caches all
relevant registers, the axp288_charger_is_present() and
axp288_charger_is_online() helpers are not necessary anymore.

Directly check the cached input_status instead, allowing the removal
of these 2 helpers.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>