Reviewers sidebar: Add ability to quickly remove reviewers

What does this MR do and why?

Adds a remove action to quickly remove reviewers.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable FF echo "Feature.enable(:reviewer_assign_drawer)" | rails c
2. Add reviewers to an MR
3. Remove a reviewer

Related to #469616 (closed), #480218 (closed)