Remove `has_many` references to `Vulnerabilities::Feedback`

At the time of writing, the following classes have has_many references to Vulnerabilities::Feedback:

• Project
• User

The vulnerability_feedback table additionally has the following foreign key references, but the referenced class do not appear to have AR relations defined referencing back to the Vulnerabilities::Feedback model:

• merge_requests
• users on comment_author_id (author_id is referenced above)
• issues