Check agent limit against root agent count rather than both root and child agents

From sl docs such as http://community.secondlife.com/t5/English-Knowledge-Base/Managing-Private-Regions/ta-p/700115
agent should apply to avatars only.
This makes sense from a user perspective, and also from a code perspective since child agents with no physics or actions take up a fraction of root agent resources.
As such, the check is now only performed in Scene.QueryAccess() - cross and teleport check this before allowing an agent to translocate.
This also removes an off-by-one error that could occur in certain circumstances on teleport when a new child agent was double counted when a pre-teleport agent update was performed.
This does not affect an existing bug where limits or other QueryAccess() checks are not applied to avatars logging directly into a region.

OpenSim/Region/Framework/Scenes/SceneGraph.cs

@@ -779,11 +779,6 @@ namespace OpenSim.Region.Framework.Scenes
             return m_scenePresenceArray;
         }
 
-        public int GetNumberOfScenePresences()
-        {
-            return m_scenePresenceArray.Count;
-        }
-
         /// <summary>
         /// Request a scene presence by UUID. Fast, indexed lookup.
         /// </summary>