removed 21d expire from Announcement model

This requires an index re-build, and the responsibility is moving to
Reeeper.

app/models/announcement.js

@@ -9,7 +9,7 @@ const mongoose = require('mongoose');
 const Schema = mongoose.Schema;
 
 const AnnouncementSchema = new Schema({
-  created: { type: Date, default: Date.now, required: true, index: -1, expires: '21d' },
+  created: { type: Date, default: Date.now, required: true, index: -1 },
   title: {
     icon: {
       class: { type: String, default: 'fa-bullhorn', required: true },