#59054021 Fix Ratings Bug.

[dessHub]

What does this PR do?
Fixes bug on rating response
Also get rid of duplicate rating serializer.

Background information
The message returned had a grammatical error.

How To Manually Test?
fetch origin
git checkout bg-rating-159816402

• Update database by running migrations.
  python manage.py migrate

• Run test coverage
  python manage.py test

• Start the server
  python manage.py runserver

• Test the endpoints on postman.

Affected Endpoints:
DELETE /api/articles//rate/
Response:

{
    "rate": {
        "response": {
            "message": [
                "Successfull."
            ],
            "avg_ratings": {
                "ratings__avg": 4
            }
        }
    }
}

Screenshot:

Relevant Pivotal tracker stories
#159816402 Fix ratings bug.

[coveralls]

Coverage increased (+0.6%) to 94.408% when pulling f2042e7 on bg-rating-159816402 into b8c4073 on develop.