[misc] migrate the health-check to the native mongo driver, drop mongojs

services/notifications/app/js/HealthCheckController.js

@@ -11,7 +11,7 @@
  * DS207: Consider shorter variations of null checks
  * Full docs: https://github.com/decaffeinate/decaffeinate/blob/master/docs/suggestions.md
  */
-const { ObjectId } = require('mongojs')
+const { db, ObjectId } = require('./mongodb')
 const request = require('request')
 const async = require('async')
 const _ = require('underscore')
@@ -19,12 +19,6 @@ const settings = require('settings-sharelatex')
 const { port } = settings.internal.notifications
 const logger = require('logger-sharelatex')
 
-const mongojs = require('mongojs')
-const Settings = require('settings-sharelatex')
-const db = mongojs(Settings.mongo != null ? Settings.mongo.url : undefined, [
-  'notifications'
-])
-
 module.exports = {
   check(callback) {
     const user_id = ObjectId()