Fix force-pushable example for the ammo driver

[Elettrotecnica]

This fixes https://c-frame.github.io/aframe-physics-system/examples/ammo/compound.html, where currently the cursor will disappear upon clicking. This is because its position will be converted from world to local on the position itself, rather than on a slack vector. I have used the force vector because it was already there.

I have also removed the dampening on the force, as this seems to not be necessary (the impulse is actually rather weak now).

[diarmidmackenzie]

Yes, this code came from #37, and is clearly bugged.