HID: wacom: Remove CONFIG_HID_WACOM_POWER_SUPPLY option

This option was ment as a safety mechanism in case the system treats the wacom
tablet battery as the main power supply. It's no longer required as now we can
distinguish between system power supply and device power supply.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Reviewed-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>