[FLINK-8171] [flip6] Remove work arounds from Flip6LocalStreamEnvironment

[tillrohrmann]

What is the purpose of the change

It is no longer needed to wait for the registration of task managers and
to not use slot sharing when submitting jobs to the Flip-6 MiniCluster.
Therefore, we can remove these work arounds from the
Flip6LocalStreamEnvironment.

This PR is based on #5091.

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)

[zentol]

Can we also remove this bit

// temporary hack for FLIP-6 since slot-sharing isn't implemented yet
jobGraph.setAllowQueuedScheduling(true);

from the RestClusterClient?

[tillrohrmann]

I think we can remove the comment but not disabling the queued scheduling mode which is mandatory for Flip-6 to work. I'll remove the comment.

[tillrohrmann]

Thanks for the review @zentol. I've rebased this PR and once Travis gives green light, I'll merge it.