[database] remove transaction listener instance from class

lib/modules/database/transaction.js

@@ -21,14 +21,13 @@ const generateTransactionId = (): number => transactionId++;
  */
 export default class TransactionHandler {
   _database: Database;
-  _transactionListener: Function;
   _transactions: { [number]: Object };
 
   constructor(database: Database) {
     this._transactions = {};
     this._database = database;
 
-    this._transactionListener = SharedEventEmitter.addListener(
+    SharedEventEmitter.addListener(
       getAppEventName(this._database, 'database_transaction_event'),
       this._handleTransactionEvent.bind(this)
     );