SXP does not cleanup operational DS nodes after removal

When a node (or an entire topo) is deleted from the Config DS, it was deleted from the operational DS only partly. I.e. its non-sxp-augmented counterparts are left untouched. This effectively creates a "DB-memory leak" : The SXP controller would write a new network topology node stub to the DS when adding an SXP node, but would not clean it up when removing the node afterwards.