Don't eagerly clear frame collision events when physics actors subscribe and unsubscribe from collisions, in order to avoid a race condition.

Since this is done directly from ScenePresence, it can lead to a race condition with the simulator loop.
There's no real point doing it anyway since the clear will be done very shortly afterwards by the simulate loop and either there are no events (for a new avatar) or events don't matter (for a departing avatar).
This matches existing behaviour in OdePrim

OpenSim/Region/Physics/OdePlugin/ODECharacter.cs

@@ -1261,14 +1261,20 @@ namespace OpenSim.Region.Physics.OdePlugin
         {
             m_requestedUpdateFrequency = ms;
             m_eventsubscription = ms;
-            CollisionEventsThisFrame.Clear();
+
+            // Don't clear collision event reporting here.  This is called directly from scene code and so can lead
+            // to a race condition with the simulate loop
+
             _parent_scene.AddCollisionEventReporting(this);
         }
 
         public override void UnSubscribeEvents()
         {
             CollisionEventsThisFrame.Clear();
-            _parent_scene.RemoveCollisionEventReporting(this);
+
+            // Don't clear collision event reporting here.  This is called directly from scene code and so can lead
+            // to a race condition with the simulate loop
+            
             m_requestedUpdateFrequency = 0;
             m_eventsubscription = 0;
         }