[hotfix] Remove unnecessary java.io.Serializable

[beliefer]

What is the purpose of the change

This PR proposes to remove unnecessary java.io.Serializable.
Since StateBackend already extends java.io.Serializable.

Brief change log

Remove unnecessary java.io.Serializable.

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)

[flinkbot]

CI report:

• 639e67c Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[beliefer]

ping @aljoscha @StephanEwen cc @1996fanrui @davidradl @GOODBOY008