[hotfix] Forbid instantiate util class Executors

[tisonkun]

What is the purpose of the change

Forbid instantiate util class Executors

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, 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)

cc @zentol

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[zentol]

There are probably hundreds of classes that we could change like this; I'd rather not spend any efforts on remedying this as the value is, well, basically non-existent.