HID: hid-multitouch: class MT_CLS_STANTUM is redundant with MT_CLS_CONFIDENCE

Stantum devices used to work with MT_CLS_STANTUM but MT_CLS_CONFIDENCE
is exactly the same. This patch switches them to this generic class,
and remove the unused MT_CLS_STANTUM.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>