remove world dependency from btDeformableBodySolver,btDeformableBackwardEulerObjective, and btCGProjection; reduce invasion into multibody world, all chnages are cosmetic now

src/BulletSoftBody/btDeformableBackwardEulerObjective.cpp

@@ -151,10 +151,6 @@ void btDeformableBackwardEulerObjective::initialGuess(TVStack& dv, const TVStack
 //set constraints as projections
 void btDeformableBackwardEulerObjective::setConstraints()
 {
-    // build islands for multibody solve
-    m_world->btMultiBodyDynamicsWorld::buildIslands();
-    // for repeated constraint solve, splitIslands has to be set to true
-    m_world->setSplitIslands(true);
     projection.setConstraints();
 }