[FLINK-11504] Remove invalid test JobManagerConnectionTest

[tisonkun]

What is the purpose of the change

This test tests really legacy classes. We now have rpc abstractions and their own tests.

Brief change log

Delete JobManagerConnectionTest.scala

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, 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)

cc @tillrohrmann

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• ❌ 1. The [description] looks good.
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[zentol]

Test is covered in org.apache.flink.runtime.rpc.akka.AkkaRpcActorTest#testFailingAddressResolution.