HID: wacom: generic: remove input_event_flag

Input_event_flag duplicates the information we track in
wacom_wac->hid_data.inrange_state for the pad.

Signed-off-by: Aaron Skomra <aaron.skomra@wacom.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>