GH-37853: [Python] Remove test and fixture involving fastparquet

[raulcd]

Rationale for this change

test_fastparquet_cross_compatibility is not exercised and when exercised with latest versions it fails because it's not compatible with pandas 3 and it won't be as the project is retired, see:

• Add retirement message dask/fastparquet#973

What changes are included in this PR?

Removal of fixture to mark test as fastparquet and remove single test that still required fastarquet.

Are these changes tested?

Yes and no. CI is exercised as expected but fastparquet tests weren't exercised on CI in the first place.

Are there any user-facing changes?

No

• GitHub Issue: [Python][CI] Tests involving fastparquet are never run #37853

[Copilot]

Pull request overview

This PR removes the last remaining fastparquet-specific pytest wiring and an unexercised cross-compatibility test that now fails with pandas 3 due to fastparquet being effectively retired. This aligns the PyArrow Python test suite with dependencies that are actually supported and exercised in CI.

Changes:

• Removed test_fastparquet_cross_compatibility from the Parquet basic test suite.
• Removed the fastparquet optional-dependency group/default detection from python/pyarrow/conftest.py.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/pyarrow/tests/parquet/test_basic.py	Removes the unused fastparquet cross-compatibility test that is incompatible with pandas 3.
python/pyarrow/conftest.py	Removes the fastparquet optional dependency group and import-based enablement since no tests require it anymore.