Remove OAuthCredentials, Repository.credentials, Gitlab providers and dependency

arkindex/process/tests/test_process_datasets.py

arkindex/process/tests/test_processes.py

arkindex/process/tests/test_providers.py

arkindex/process/tests/test_repos.py

arkindex/process/tests/test_signals.py

arkindex/process/tests/test_worker_configurations.py

arkindex/process/tests/test_workerruns.py

arkindex/process/tests/test_workers.py

arkindex/project/api_v1.py

arkindex/project/checks.py

arkindex/project/settings.py

arkindex/project/tests/test_checks.py

arkindex/project/urls.py

arkindex/users/admin.py

arkindex/users/api.py

arkindex/users/migrations/0001_initial.py

arkindex/users/migrations/0003_delete_oauthcredentials.py

arkindex/users/models.py

arkindex/users/providers.py

arkindex/users/serializers.py