HID: multitouch: ditch mt_report_id

Now that the driver can handle more than one multitouch collection in
a single HID device, ditch the last bit that contains us to use only
one mt collection.

Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>