Remove cred_files migration

[metasploit_data_models 0.4.0](https://github.com/rapid7/metasploit_data_models/pull/7) removed Mdm::CredFile as it was only used in Metasploit Pro, so the model has been removed from Mdm.  The migration has been moved to Pro. 

Coordinated merge tasks:
- [x] Complete all tasks for https://github.com/rapid7/metasploit_data_models/pull/7
- [x] `bundle install` in this PR's branch so `Gemfile.lock` references the new commit sha for tag 0.4.0
- [ ] Merge this pull request