Removed unused Bluebird import

refs https://github.com/TryGhost/Ghost/issues/14882

- this is no longer used so we can remove it as we move towards removing
  Bluebird from the codebase

ghost/core/core/server/models/user.js

@@ -1,5 +1,4 @@
 const _ = require('lodash');
-const Promise = require('bluebird');
 const validator = require('@tryghost/validator');
 const ObjectId = require('bson-objectid').default;
 const ghostBookshelf = require('./base');