FLINK-36479 remove deprecated method Table getSchema as

[jbalchan]

What is the purpose of the change

[FLINK-36479] (https://issues.apache.org/jira/browse/FLINK-36479)
Component: Table SQL / API

Remove deprecated methods Table getSchema and as

Brief change log

Remove deprecated methods Table getSchema and as

Verifying this change

This change is already covered by existing tests

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:

• 2301bb6 Azure: FAILURE

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[davidradl]

I see a test that uses this function in Scala in OverWindowValidationTest is has .
Over.partitionBy($"c").orderBy($"rowtime").preceding(2).following($"xx").as($"w"))