Get rid of btSolverBody and use btRigidBody directly. btSolverBody didn't improve performance after all, due to random-access

Tweak the BenchmarkDemo a bit: 

1) disable deactivation in graphical mode
2) add some settings that increase performance in the BenchmarkDemo2 (1000 stack) from 35ms to 15ms on this quad core (at the cost of a bit of quality)

src/BulletDynamics/ConstraintSolver/btContactConstraint.cpp

@@ -56,10 +56,6 @@ void	btContactConstraint::buildJacobian()
 
 }
 
-void	btContactConstraint::solveConstraintObsolete(btSolverBody& bodyA,btSolverBody& bodyB,btScalar	timeStep)
-{
-
-}