Complete the parallel solver fix

src/dynamics/joint/impulse_joint/impulse_joint.rs

@@ -15,5 +15,4 @@ pub struct ImpulseJoint {
 
     // A joint needs to know its handle to simplify its removal.
     pub(crate) handle: ImpulseJointHandle,
-    pub(crate) constraint_index: usize,
 }

src/dynamics/joint/impulse_joint/impulse_joint_set.rs

@@ -187,7 +187,6 @@ impl ImpulseJointSet {
             data,
             impulses: na::zero(),
             handle: ImpulseJointHandle(handle),
-            constraint_index: 0,
         };
 
         let default_id = InteractionGraph::<(), ()>::invalid_graph_index();