chore(db): remove unused mysql models barrel

models/mysql/index.js was never imported by the application — mysql.js
is the actual model registry. The barrel created a false registration
point that caused the Application migration bug.

src/database/models/mysql/index.js

@@ -1,18 +0,0 @@
-exports.Application = require('./Application');
-exports.Article = require('./Article');
-//exports.ArticleTag = require('./ArticleTag');
-exports.Brand = require('./Brand');
-exports.Category = require('./Category');
-exports.Comment = require('./Comment');
-exports.Favorite = require('./Favorite');
-exports.Follower = require('./Follower');
-exports.Packaging = require('./Packaging');
-exports.Product = require('./Product');
-exports.ProductCategoryXref = require('./ProductCategoryXref');
-exports.ProductTagXref = require('./ProductTagXref');
-exports.Role = require('./Role');
-exports.SkydiverProfile = require('./SkydiverProfile');
-exports.Tag = require('./Tag');
-exports.TagList = require('./TagList');
-exports.User = require('./User');
-exports.UserRoleXref = require('./UserRoleXref');