Remove stale TODO comments for XCom map_index handling

[fat-catTW]

Why

Hello

The existing implementation already passes map_index as a query parameter.

I verified that this matches the current Execution API route:

• airflow-core/src/airflow/api_fastapi/execution_api/routes/xcoms.py

and is consistent with the OpenAPI specification for the XCom endpoints. Since the implementation is already correct, the TODO comments are no longer necessary.

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.