Removes debug printouts in sprite physics

examples/game/test_physics.dart

@@ -85,7 +85,6 @@ class TestBed extends NodeWithSize {
   }
 
   void myCallback(PhysicsContactType type, PhysicsContact contact) {
-    print("CONTACT type: $type");
   }
 
   bool handleEvent(SpriteBoxEvent event) {

packages/flutter_sprites/lib/src/node.dart

@@ -155,7 +155,7 @@ class Node {
       return _physicsWorld(parent.parent);
     }
     else {
-      print("physics not found");
+      assert(false);
       return null;
     }
   }