Tweak terrain collision detect logic

Running `collisionDetect` with `active_collision_options.terrain_only` set to
`true` now implies that you want to check for collisions against the `terrain`
group. Or rather, it always implied that, but now the code actually respects
that.
Remove no-longer-necessary hack from QPController.

Takes out a bit of code that was needed because `collisionDetect` didn't
respect filtering by group for terrain collisions.