[FLINK-37575] Simplify the code for creating Netty shuffle environment.

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What is the purpose of the change

This PR aims to simplify the code for creating Netty shuffle environment.
Currently, NettyShuffleServiceFactory.createNettyShuffleEnvironment has a parameter 'String[] tmpDirPaths', but its true purpose is only to use its length.

Even if developers want to use the value of String [] tmpDirPaths in the future, it's no problem due to the parameter NettyShuffleEnvironmentConfiguration config of NettyShuffleServiceFactory.createNettyShuffleEnvironment has the field private final String[] tempDirs; too. We can use it by calling the getTempDirs show below.

    public String[] getTempDirs() {
        return tempDirs;
    }

This PR also unify the temp directory with tempDirs.

Brief change log

Simplify the code for creating Netty shuffle environment.

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)

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CI report:

• fa26298 Azure: SUCCESS

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