[FLINK-16257][network] Remove useless ResultPartitionID from AddCredit message

[zhijiangW]

What is the purpose of the change

The ResultPartitionID in AddCredit message is never used on upstream side, so we can remove it to cleanup the codes. There would have another two benefits to do so:

1. Reduce the total message size from previous 52 bytes to 20 bytes.
2. Decouple the dependency with InputChannel#getPartitionId.

Brief change log

• Remove ResultPartitionID from AddCredit message

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): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

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Last check on commit 75bddc5 (Wed Feb 26 05:51:09 UTC 2020)

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[rkhachatryan]

nit: replace numbers with constants Integer.BYTES and Long.BYTES?

[zhijiangW]

Yes, it seems more readable for this change, but not quite relevant with this PR purpose.
After double checking the codes, there are many other places to take the current way in this class. So I might consider this change as a separate issue for unification future.

[rkhachatryan]

LGTM except for one optional thing (see above)

[zhijiangW]

Thanks for the review @rkhachatryan , I replied the inline comments above.