[FLINK-22832][sql-client] Drop usages of legacy planner in SQL Client

[twalthr]

What is the purpose of the change

This removes support of the legacy planner in SQL Client. It also removes the concept of having different planners. A user will get a helpful exception if table.planner=old is used. table.planner=blink is still supported.

Brief change log

• remove all code locations to legacy planner
• remove BatchTableEnvironment

Verifying this change

This change is already covered by existing tests. Also the change has been manually verified.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): yes
• The public API, i.e., is any changed class annotated with @Public(Evolving): yes
• 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/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

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Automated Checks

Last check on commit d5bc63c (Sat Aug 28 12:14:14 UTC 2021)

Warnings:

• 1 pom.xml files were touched: Check for build and licensing issues.

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[twalthr]

CC @fsk119

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CI report:

• d5bc63c Azure: SUCCESS

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[fsk119]

Thanks for your great work!

LGTM.