[FLINK-33678][streaming-java] Remove RestartStrategies related public API.

[JunRuiLee]

What is the purpose of the change

FLINK-33581/FLIP-381 Deprecate configuration getters/setters that return/set complex Java objects. In Flink 2.0 we should remove these deprecated method and fields. This change will prevent users from configuring their jobs by passing complex Java objects, encouraging them to use ConfigOption instead.

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, 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)