[firestore] remove transaction listener instance from class

lib/modules/firestore/TransactionHandler.js

@@ -37,13 +37,12 @@ type TransactionEvent = {
  */
 export default class TransactionHandler {
   _firestore: Firestore;
-  _transactionListener: Function;
   _pending: { [number]: TransactionMeta };
 
   constructor(firestore: Firestore) {
     this._pending = {};
     this._firestore = firestore;
-    this._transactionListener = SharedEventEmitter.addListener(
+    SharedEventEmitter.addListener(
       getAppEventName(this._firestore, 'firestore_transaction_event'),
       this._handleTransactionEvent.bind(this)
     );