refactor: clarify callback responsibility and remove redundant topic cleanup logic

[BackendArchitectX]

What changed

• Remove the redundant deleted-topic traversal from ElasticReplicaManager#asyncApplyDelta.
• Preserve callbacks for actual local partition deletion and partition state transitions.
• Document the callback's immediate per-partition notification responsibility.

Why

Global deleted-topic cleanup is already handled by BrokerMetadataPublisher#notifyGroupCoordinatorOfDeletedPartitions after asyncApplyDelta completes. The additional deletedTopicIds() traversal in ElasticReplicaManager therefore caused redundant cleanup notifications.

Fixes #2721

Testing

• :core:compileScala — passed
• :core:S3UnitTest --tests unit.kafka.server.streamaspect.ElasticReplicaManagerTest — passed