Remove redundant OpenAPI CI job

The SchemaView unit tests already take care of testing the full schema generation;
this job wastes some CI time doing what the tests already do and does not provide
anything more than what a unit test can do.

.gitlab-ci.yml

@@ -59,17 +59,6 @@ backend-migrations:
     - pip install -e .
     - arkindex/manage.py makemigrations --check --noinput --dry-run -v 3
 
-backend-openapi:
-  extends: .backend-setup
-  stage: build
-
-  script:
-    - ci/openapi.sh
-
-  artifacts:
-    paths:
-      - output/
-
 backend-static:
   extends: .backend-setup
   stage: build