Create a service for deleting review assignments

[mkimberlin]

As a Manager, I want to delete a review assignment I created in case of an error or if the review is no longer needed so that I can keep my review list accurate and avoid clutter.

Acceptance Criteria:

• The service allows a DELETE request to an endpoint like /reviews/assignments/{id} where {id} is the unique identifier for the assignment.
• The service deletes the specified review assignment.
• The service returns a success message upon successful deletion.