HID: wacom: remove unused packet lengths

We use generic hid_report_len() to get individual packet length now.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>