love.physics: move internal box2d->love object map to World instances, and clean up some physics object code. Resolves issue #1465.

src/modules/physics/box2d/Fixture.h

@@ -38,6 +38,8 @@ namespace physics
 namespace box2d
 {
 
+class World;
+
 /**
  * This struct is stored in a void pointer
  * in the Box2D Fixture class. For now, all we
@@ -68,11 +70,6 @@ public:
 	 **/
 	Fixture(Body *body, Shape *shape, float density);
 
-	/**
-	 * Creates a Fixture.
-	 **/
-	Fixture(b2Fixture *f);
-
 	virtual ~Fixture();
 
 	/**