[misc] mongodb: drop intermediate dbPromise

It would confuse the logic of a follow-up commit.

services/chat/app/js/mongodb.js

@@ -2,10 +2,9 @@ const Settings = require('settings-sharelatex')
 const { MongoClient, ObjectId } = require('mongodb')
 
 const clientPromise = MongoClient.connect(Settings.mongo.url)
-const dbPromise = clientPromise.then((client) => client.db())
 
 async function getCollection(name) {
-  return (await dbPromise).collection(name)
+  return (await clientPromise).db().collection(name)
 }
 
 async function waitForDb() {