gather verb should clean dtmf listerner even dtmfBargein=false (#686)

lib/tasks/gather.js

@@ -965,10 +965,8 @@ class TaskGather extends SttTask {
     // If bargin is false and ws application return ack to verb:hook
     // the gather should not play any audio
     this._killAudio(this.cs);
-    // Clear dtmf event
-    if (this.dtmfBargein) {
-      this.ep.removeAllListeners('dtmf');
-    }
+    // Clear dtmf events, to avoid any case can leak the listener, just clean it
+    this.ep.removeAllListeners('dtmf');
     clearTimeout(this.interDigitTimer);
     this._clearTimer();
     this._clearFastRecognitionTimer();