[FLINK-32965][flink-runtime] Removing the deprecated MutableObjectMode and restore related tests

[1996fanrui]

What is the purpose of the change

FLINK-1285 updated the MutableObjectMode to ObjectReuse. The TaskConfig#getMutableObjectMode() is not used for a long long time, and it's internal class, so it can be removed directly.

Also, we should update the GroupReduceDriverTest:

• Updating the testAllReduceDriverAccumulatingImmutable() from context.setMutableObjectMode(false); to context.getExecutionConfig().disableObjectReuse();.
• Updating the testAllReduceDriverIncorrectlyAccumulatingMutable() to context.getExecutionConfig().disableObjectReuse();, and fix the assert.

More details can be get from #23233 (comment)

Brief change log

• [FLINK-32965][flink-runtime] Removing the deprecated MutableObjectMode and restore related tests
• [FLINK-32965][flink-runtime][hotfix] Refactor some code with code style warning

Verifying this change

Improve the old tests:

• GroupReduceDriverTest#testAllReduceDriverIncorrectlyAccumulatingMutable
• GroupReduceDriverTest#testAllReduceDriverAccumulatingImmutable

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

[1996fanrui]

Hi @Jiabao-Sun @RocMarshal would you mind helping review this PR in your free time? thanks~

[flinkbot]

CI report:

• ba6d6da Azure: SUCCESS

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

[ferenc-csaky]

Thanks @1996fanrui for this change, LGTM!

[RocMarshal]

Hi, @1996fanrui
Thanks for the contribution and ping.
LGTM +1.

[1996fanrui]

Thanks @ferenc-csaky and @RocMarshal for the review, merging~