HID: wacom: power_supply: remove ac information

Looks like upowerd is ignoring this since October 2013, so there is
no need to keep this around in the kernel.
And as mentioned in 8aaa592 (linux: Ignore ACs coming from devices) in
the upower tree, "We already have enough information on the device
battery".

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>