power: supply: axp288_fuel_gauge: Remove debugfs support

The debugfs code is simply just dumping a bunch of registers, the same
information can also easily be gotten through the regmap debugfs
interface or through the i2cdump utility.

I've not used the debugfs interface once in all these years that I've
been working on the axp288_fuel_gauge driver, so lets just remove it.

Note this also removes the temperature-channels from the list of
IIO ADC channels used by the driver, since these were only used in the
debugfs interface.

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