Clean-up the picking code a little bit

We only have one layer, so remove code that checks if there's more
layers. Also remove weird pointer and reference usage.

Change-Id: Ia8042f03950d4f1c29246583a5820c4e07a7533e
Reviewed-by: Antti Määttä <antti.maatta@qt.io>
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>