chore(models): remove dead MongoDB Article, Comment, Tag models

Migration to MySQL is complete for these three entities — ArticleService,
CommentService, and TagService all use Sequelize exclusively. No remaining
references to the Mongoose versions anywhere in the codebase.

src/database/models/mongo/index.js

@@ -10,9 +10,6 @@ exports.QcmCategory = require('./QcmCategory');
 exports.QcmQuestion = require('./QcmQuestion');
 exports.QcmChoice = require('./QcmChoice');
 exports.X2DataLog = require('./X2DataLog');
-exports.Article = require('./Article');
-exports.Comment = require('./Comment');
-exports.Tag = require('./Tag');
 exports.HWClan = require('./herowars/Clan');
 exports.HWMember = require('./herowars/Member');