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.cpp

@@ -4490,10 +4490,6 @@ void entityclass::applyfriction( int t, float xrate, float yrate )
     if (std::abs(entities[t].vy) < yrate) entities[t].vy = 0.0f;
 }
 
-void entityclass::cleanup()
-{
-}
-
 void entityclass::updateentitylogic( int t )
 {
     entities[t].oldxp = entities[t].xp;