[hotfix] Reuse the final field barrierId

[beliefer]

What is the purpose of the change

This PR aims to reuse the final field barrierId in CheckpointBarrierTracker. Because the barrierId is a final field, we can reuse it safely.

Brief change log

Reuse the final field barrierId.

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 documented)

[flinkbot]

CI report:

• 97d2612acf94a0e9e7b1d065dbccc32b89928339 Azure: SUCCESS

Bot commands

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

[beliefer]

ping @gaoyunhaii @AHeise @zhijiangW @pnowojski cc @1996fanrui @davidradl

[beliefer]

@ferenc-csaky @davidradl Thank you!