[FLINK-11833] [State Backends] Cleanup unnecessary createKeyedStateBackend methods in StateBackend

[carp84]

What is the purpose of the change

This PR is a simple cleanup of StateBackend interface

Brief change log

Removed unnecessary createKeyedStateBackend methods and completed the javadoc for the left one.

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): (yes)
  • Removed two methods from StateBackend that are only used in tests.
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, 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)

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[carp84]

Checked the travis build and confirmed the failure is irrelative to change here. Verified in local travis build and it passed w/o failure.

[StefanRRichter]

Changes LGTM and the build failure seems unrelated 👍 Nice job! Will merge.

[carp84]

Thanks Stefan!