[FLINK-28109][connector/elasticsearch] Delete useful code in the row emitter

[a49a]

Brief change log

• Delete id(key) in the RowElasticsearchEmitter class

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[flinkbot]

CI report:

• 040d7e2 Azure: FAILURE

Bot commands

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• @flinkbot run azure re-run the last Azure build

[MartijnVisser]

Needs to be moved to https://github.com/apache/flink-connector-elasticsearch