If you already have a "forward" 14-pin harness (like Metra 70-8901 or equivalent), that's fine -- you don't need mine.
To extract the unneeded speaker wires, open the hinged locks near the back (where the wires exit), and use a flat blade jeweler's screwdriver (like for eyeglasses) to release a plastic spring clip holding each pin. Cross section of a similar connector:
(click for larger image)
I don't recommend trying to extract the wires on the Pioneer's harness.