Add MergeRequestReviewer for dedicated Reviewers section

What does this MR do?

Related to #216054 (closed)

This is the first step towards adding dedicated reviewer section to MergeRequests. It'll be available for EE and I'll add a feature flag in controller level so I don't think we need documentation update at this stage.

FYI, this is the next WIP MR that uses this model. !40378 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

app/models/merge_request_reviewer.rb

@@ -3,6 +3,4 @@
 class MergeRequestReviewer < ApplicationRecord
   belongs_to :merge_request
   belongs_to :reviewer, class_name: "User", foreign_key: :user_id, inverse_of: :merge_request_assignees
-
-  validates :reviewer, uniqueness: { scope: :merge_request_id }
 end