Delete necessary records when repositories are unsubscribed from [Curry]

:fork_and_knife: :curry: When unsubscribing from a Curry::Repository, delete the associated records that should be cleaned up.

List of models that need to get cleaned up when a Chef admin unsubscribes from a repo:

- [x] Curry::PullRequest - the pull requests for that repo
- [x] Curry::PullRequestCommitAuthor - the join table records for the pull requests for that repo
- [x] Curry::PullRequestUpdates - the pull request updates for the pull requests for that repo

To handle:

- [x] What if a worker is kicked off and the pull request no longer exists?

This spawned from and closes https://app.getsentry.com/chef/supermarket/group/15082972/