physics: Slight code cleanup in PhysicalNode::clear()

panda/src/physics/physicalNode.I

@@ -16,12 +16,11 @@
  */
 INLINE void PhysicalNode::
 clear() {
-  PhysicalsVector::iterator it;
-  for (it = _physicals.begin(); it != _physicals.end(); ++it) {
-    nassertd((*it)->_physical_node == this) continue;
-    (*it)->_physical_node = nullptr;
+  for (Physical *physical : _physicals) {
+    nassertd(physical->_physical_node == this) continue;
+    physical->_physical_node = nullptr;
   }
-  _physicals.erase(_physicals.begin(), _physicals.end());
+  _physicals.clear();
 }
 
 /**