Avoid importing dask-expr if "query-planning" config is False

[rjzamora]

Description

During some offline debugging with @bdice and @divyegala, we discovered that some cuml tests are somehow failing after dask_expr is imported - Even if dask_expr is not actually being used. I'd like to figure out exactly what is causing that problem, but the first thing we can/should do is avoid the import altogether when the "query-planning" config is set to False.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

This appears to fix the failing cuml test for me.

Ran with:

python -m pytest --cache-clear test_dask_nearest_neighbors.py::test_compare_skl[5-True-1-10-5-10-300] -s -v

I think this will resolve rapidsai/cuml#5803.

[rjzamora]

/merge