Replace pytest-celery with `celery.contrib.pytest` plugin loading

The new 1.0.0 version of pytest-celery will move towards a Docker-based
approach for smoke and production-like testing, bringing in a number of
dependencies that we don't currently need.
Since the pre-1.0.0 version of pytest-celery was just a shim for
``celery.contrib.pytest``, we can simply load the latter for API,
integration and Selenium tests.

test/integration/conftest.py

@@ -2,7 +2,10 @@ import tempfile
 
 import pytest
 
-from galaxy_test.conftest import pytest_configure  # noqa: F401
+from galaxy_test.conftest import (  # noqa: F401
+    pytest_configure,
+    pytest_plugins,
+)
 
 
 @pytest.fixture(scope="session")