Removed empty constructors for constraints, I don't recall why we added them (possibly SPU) but if we really need them we have to revert this.

Moved static fixed body into a static function 'getFixedBody' to avoid issues when embedding Bullet in C#/CLI managed code.

src/BulletDynamics/ConstraintSolver/btHingeConstraint.cpp

@@ -30,16 +30,7 @@ subject to the following restrictions:
 
 #ifndef __SPU__
 
-btHingeConstraint::btHingeConstraint()
-: btTypedConstraint (HINGE_CONSTRAINT_TYPE),
-m_enableAngularMotor(false),
-m_useSolveConstraintObsolete(HINGE_USE_OBSOLETE_SOLVER),
-m_useOffsetForConstraintFrame(HINGE_USE_FRAME_OFFSET),
-m_flags(0),
-m_useReferenceFrameA(false)
-{
-	m_referenceSign = m_useReferenceFrameA ? btScalar(-1.f) : btScalar(1.f);
-}
+