Fix dask-cudf read_parquet bug for multi-file aggregation

[rjzamora]

Description

Fixes a subtle bug in CudfEngine.read_partition, and removes stale code in test_parquet.py.
One motivation for this PR is to avoid CI failures if/when dask/dask#9637 is merged in upstream Dask.

Checklist

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

[wence-]

Thanks, looks good!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-23.04@5b278b1). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

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[rjzamora]

/merge