[FLINK-15668][tests] Remove travis test stages for legacy scheduler

[zhuzhurk]

What is the purpose of the change

AlsoRunWithLegacyScheduler is used to enable IT cases for legacy scheduler.
This task is to remove it as well as the related test stages from travis.

Brief change log

• Removed AlsoRunWithLegacyScheduler
• Removed test stages for legacy scheduler

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): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Automated Checks

Last check on commit ea341af (Fri Oct 16 10:32:56 UTC 2020)

Warnings:

• 1 pom.xml files were touched: Check for build and licensing issues.
• No documentation files were touched! Remember to keep the Flink docs up to date!

_{Mention the bot in a comment to re-run the automated checks.}

Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• ea341af Azure: FAILURE

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[zhuzhurk]

@tillrohrmann @GJL would you help to review this change when you have time?

[tillrohrmann]

Thanks for creating this PR @zhuzhurk. I talked to @GJL offline and we concluded that we would first like to release the new scheduler and gather a bit of feedback from users before we remove the legacy code. This effectively means that we will review and merge this PR in a bit and not right now.

[tillrohrmann]

Thanks for creating this PR @zhuzhurk. LGTM. I will rebase and resolve the merge conflicts first. If AZP gives green light, then I will merge this PR.

[tillrohrmann]

Failing e2e test case is unrelated. Merging this PR now.