Remove code allowing missing models to be instantiated via a base class.

Rails 4.2 doesn't like this monkey patch since instantiate's arity changed from Rails 3.2 (1) to Rails 4.2(2).
We can do this better by using a migration to delete miq_workers rows with the offending values.

When we delete models, we need to write a data migration to remove rows for that model.
Remove previously deleted MiqWorker sublcasses from miq_workers table.
Merge pull request #1266 from jrafanie/remove_clever_code

Remove clever code (AR::Base instantiate)