[FLINK-17622][connectors / jdbc] Remove useless switch for decimal in PostgresCatalog

[fpompermaier]

What is the purpose of the change

This pull request remove 2 useless constants and switch clauses in the PostgresCatalog class.

Brief change log

The decimal fields are returned as numeric by the Postgres JDBC driver. The code to verify this fact are introduced in the pull request #11900.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.
However serial type is auto-incremented: you can write a value into it but if you try to autogenerate a value you get duplicate values until you find a value that is not already in use. However Flink does not keep track of the original field type so there's no way to warn the user that is not using this field correctly (he/she should use DEFAULT or not specify any value for such columns).

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/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 documented

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Automated Checks

Last check on commit 29252f2 (Fri Oct 16 10:51:55 UTC 2020)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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CI report:

• 29252f2 Azure: SUCCESS

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[bowenli86]

Hi @fpompermaier , can you rebase the PR?

[fpompermaier]

Cone on guys...this is a really easy PR and it's open since 3 weeks..I don't want ro rebase again because of the other PR that is moving dbc dialects in dedicated classes (one for every dialect)...are you kidding me??

[leonardBang]

Thanks @fpompermaier for the contribution, generally LGTM, only a note comment.
I think we can update it and merge soon.

[fpompermaier]

I addressed your comment

[leonardBang]

+1 to merge cc @wuchong

[wuchong]

LGTM. I added a reference to the postgres jdbc type definition to make the mapping more understandable.

Waiting for the green build.

[wuchong]

Passed.
Merging...