Remove leftovers from #45184 clear_not_launched_queued_tasks removal

[FrankYang0529]

#45184 removed clear_not_launched_queued_tasks() (switching from the proactive stuck-QUEUED sweep to internal retries on pod deletion) but left two pieces of its supporting machinery behind:

• KubernetesExecutor.last_handled — written on every task launch (execute_async) but never read or cleared anywhere, since both its only reader and its only del lived inside the removed method. It grows by one entry per launched task for the scheduler's lifetime — an unbounded leak that serves no purpose.
• The kubernetes_executor.clear_not_launched_queued_tasks.duration metric in the registry YAML — its Stats.timer(...) was removed with the method, so the catalog now lists a metric that is never emitted.

related: #45184

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[arkadiuszbach]

Just spent few hours analyzing this memory leak in Scheduler on Airflow 3.2.2

[FrankYang0529]

Thanks for the approval, @Lee-W 🙏

Since this touches the KubernetesExecutor scheduler path and fixes an unbounded memory leak, would another committer be willing to give it a second look? Thank you.

[jason810496]

Nice catch!