Fixed #20503 - Moved doctest utilities in with the rest of the deprecated test code.

The ``DocTestRunner`` and ``OutputChecker`` were formerly in
``django.test.testcases``, now they are in ``django.test.simple``. This avoids
triggering the ``django.test._doctest`` deprecation message with any import
from ``django.test``. Since these utility classes are undocumented internal
API, they can be moved without a separate deprecation process.

Also removed the deprecation warnings specific to these classes, as they are
now covered by the module-level warning in ``django.test.simple``.

Thanks Anssi for the report.

Refs #17365.