Fixed pytest marker warnings by removing unused pytest.ini

[TomAugspurger]

The CI for raft-dask includes warnings about unregistered pytest markers: https://github.com/rapidsai/raft/actions/runs/13394517078/job/37411078661#step:10:2656

../../../pyenv/versions/3.12.9/lib/python3.12/importlib/__init__.py:90
  /pyenv/versions/3.12.9/lib/python3.12/importlib/__init__.py:90: PytestUnknownMarkWarning: Unknown pytest.mark.mg - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/stable/how-to/mark.html
    return _bootstrap._gcd_import(name[level:], package, level)

Those markers are registered here at the root of raft-dask, but a second pytest.ini in raft_dask/tests/pytest.ini shadowed that pytest.ini. Without the second pytest.ini, I don't see these warnings locally.