Fix misleading log message in task_runner clear_on_success block

[amoghrajesh]

---

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

• Yes (please specify the tool below)

Raised by @kaxil on PR #66859 post review comment.

The logs mention that "Task state will be cleared by the server because clear_on_success is enabled."

But _clear_backend_only() is called immediately after on the worker side, the backend
(e.g. custom S3 store) is cleared here, not by the server. Only the DB rows are cleared by
the server.

---

• 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.