[FLINK-38027][table] Simplify watermark expressions

[snuyanzin]

What is the purpose of the change

The PR adds simplify call for watermark expressions

Verifying this change

SourceWatermarkTest.scala

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)
• If yes, how is the feature documented? (not applicable )

[flinkbot]

CI report:

• d82835f Azure: SUCCESS

Bot commands

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

[liuyongvs]

LGTM +1

[RocMarshal]

LGTM +1.