NoBackhaulEPCHelper explicitly disposes nodes when helper destructor is called

The NoBackhaulEPCHelper class has explicit calls to Dispose() on nodes in the destructor.

This causes failures if the helper is deleted before the Simulator run is invoked. For example if the the helper is built in a setup method called by main rather than in main.

This patch removes the explicit dispose method calls; leaving the disposal to the standard reference counting mechanism.