Remove reference in Swap.h to the tree-based containers, which no longer exist.

Source/Engine/Container/Swap.h

@@ -28,7 +28,6 @@ namespace Urho3D
 class HashBase;
 class ListBase;
 class String;
-class TreeBase;
 class VectorBase;
 
 /// Swap two values.
@@ -42,7 +41,6 @@ template<class T> inline void Swap(T& first, T& second)
 template<> void Swap<String>(String& first, String& second);
 template<> void Swap<VectorBase>(VectorBase& first, VectorBase& second);
 template<> void Swap<ListBase>(ListBase& first, ListBase& second);
-template<> void Swap<TreeBase>(TreeBase& first, TreeBase& second);
 template<> void Swap<HashBase>(HashBase& first, HashBase& second);
 
 }