Input: wacom - cleanup multitouch code when touch_max is 2

Historically we dealt with touch_max equals to 2 differently from
other MT devices. Now we use input_mt_*() to process all MT events,
as long as touch_max is greater than 1. So, there is no need to
take (touch_max == 2) as a special case any more.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>