Fixes pytest CI error (#6970)

```
  /var/lib/awx/venv/awx/lib64/python3.11/site-packages/_pytest/python.py:163:
  PytestReturnNotNoneWarning: Expected None, but
  awx/main/tests/unit/test_tasks.py::TestJobCredentials::test_custom_environment_injectors_with_boolean_extra_vars
  returned ['successful', 0], which will be an error in a future version
  of pytest.  Did you mean to use `assert` instead of `return`?
```

* Dug into the git blame for this one
  060585434a is the commit for any
  historians. It was wrongfully carried over from a mock pexpect
  implementation. Our new tests are nice. They don't go as far as trying
  to run the task so they do not need to mock pexpect. That is why it is
  safe to remove this code without finding it a new home.

Co-authored-by: Chris Meyers <chris.meyers.fsu@gmail.com>

awx/main/tests/unit/test_tasks.py

@@ -1259,7 +1259,6 @@ class TestJobCredentials(TestJobExecution):
         extra_vars = parse_extra_vars(args, private_data_dir)
 
         assert extra_vars["turbo_button"] == "True"
-        return ['successful', 0]
 
     def test_custom_environment_injectors_with_nested_extra_vars(self, private_data_dir, job, mock_me):
         task = jobs.RunJob()