Invoke "remove" middleware on DELETE requests

Contrary to POST and PUT request that invoke "save" middleware on Mongoose documents, when doing DELETE request the remove middleware is not called. It is because documents are removing using a static Model#remove method, which is in fact a direct MongoDB query, not involving any Mongoose documents (see: http://mongoosejs.com/docs/api.html#model_Model.remove)

The solution: Instead of removing documents with Model#remove method, first retrieve them from DB and then call the remove method on those documents instances.