Delete code in ee/app/models/vulnerabilities/feedback.rb now that we're on Rails 5.2

Summary

From the comments here, Vulnerabilities::Feedback#validate_enums and calls to it can be deleted now that we're on Rails 5.2. See original MR and discussion !10016 (merged)

Remove comment mentioning deleting code once we move to Rails 5.2

Improvements

We get to delete code that is no longer needed!

Risks

Invalid enum values would not be handled properly. We have tests in place to mitigate this problem.

Involved components

Changes will be limited to ee/app/models/vulnerabilities/feedback.rb

Optional: Intended side effects

Optional: Missing test coverage