chore(mongo-client): remove unused import

Author: mbroadst

lib/mongo_client.js

@@ -4,7 +4,6 @@ const ChangeStream = require('./change_stream');
 const Db = require('./db');
 const EventEmitter = require('events').EventEmitter;
 const executeOperation = require('./utils').executeOperation;
-const handleCallback = require('./utils').handleCallback;
 const inherits = require('util').inherits;
 const MongoError = require('mongodb-core').MongoError;
 const deprecate = require('util').deprecate;