GH-38591: [Parquet][C++] Remove redundant open calls in ParquetFileFormat::GetReaderAsync

[eeroel]

Rationale for this change

There were duplicate method calls causing extra I/O operations, apparently unintentional from 0793432.

What changes are included in this PR?

Remove the extra method calls.

Are these changes tested?

Are there any user-facing changes?

• Closes: [Parquet][Python] Potential regression in Parquet parallel reading #38591

[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}

In the case of PARQUET issues on JIRA the title also supports:

PARQUET-${JIRA_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - How to contribute patches

[github-actions]

⚠️ GitHub issue #38591 has been automatically assigned in GitHub to PR creator.

[mapleFU]

Will merge in one day if don't have any further comments

[mapleFU]

Also cc @raulcd would this better in 14.0.1?

[raulcd]

Also cc @raulcd would this better in 14.0.1?

14.0.1 is frozen and ready to be shipped

[jorisvandenbossche]

I labeled the issue as backport-candidate, though, in case we do another bug fix release

[mapleFU]

@jorisvandenbossche Maybe this should also related to #38466 and #38437 ? I'll drive them to be merged this week

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 5 benchmarking runs that have been run so far on merge-commit e62ec62.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 2 possible false positives for unstable benchmarks that are known to sometimes produce them.