remove dlg event handlers on destroy

lib/call-session.js

@@ -277,6 +277,8 @@ class CallSession extends Emitter {
           }).catch((err) => this.logger.error({err}, 'Error writing cdr for completed call'));
         }
         /* de-link the 2 Dialogs for GC */
+        dlg.removeAllListeners();
+        other.removeAllListeners();
         dlg.other = null;
         other.other = null;
         this.srf.endSession(this.req);