Remove entityclass::cleanup()

This function's sole purpose was to make sure obj.nentity was in sync,
and that obj.nentity-1 pointed to the last 'active' entity in
obj.entities. But now that obj.nentity is removed and we use
obj.entities.size() instead, it is no longer necessary.

desktop_version/src/Entity.h

@@ -163,8 +163,6 @@ public:
 
     void applyfriction(int t, float xrate, float yrate);
 
-    void cleanup();
-
     void updateentitylogic(int t);