MINOR: [Python] Remove impossible TODO in server middleware

[HyukjinKwon]

Rationale for this change

Remove a TODO comment that has been impossible to implement. The TODO asked to "prevent duplicate keys" in _ServerMiddlewareFactoryWrapper.start_call(), but this is already guaranteed by Python dict semantics. The TODO was added in commit d1848c8.

What changes are included in this PR?

Remove the TODO comment from python/pyarrow/_flight.pyx. The factories attribute has always been typed as dict (both in Cython cdef and the __init__ type hint), which inherently prevents duplicate keys.

Are these changes tested?

I did not test.

Are there any user-facing changes?

No.

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[lidavidm]

Thanks for catching this.

[lidavidm]

CI failure is #48795.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 81e4046.

There weren't enough matching historic benchmark results to make a call on whether there were regressions.

The full Conbench report has more details.