Allow user unlinking a project

What does this MR do and why?

With these changes, the policy project selector now allows a user unlinking an already assigned project to the policy editor.

Changelog: added EE: true

Screenshots or screen recordings

Implementation

Desired Interaction

https://gitlab.com/groups/gitlab-org/-/uploads/4af50b277993aaa490da968b549317f7/unlink-project.gif

How to set up and validate locally

1. Go to your project
2. Expand Security & Compliance
3. Click on Policies tab
4. Link a project
5. Unlink a project

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #344881 (closed)