[FLINK-33578][ci] Removes scala-2.12 system property

[XComp]

What is the purpose of the change

We're only relying on Scala 2.12 (and the plan is to drop it entirely for 2.x). The Scala prefix is still present in CI, though. This commit cleans it up entirely.

This clean up step helps reduce the noise in the GHA workflow change (FLINK-27075). There's no need to label workflows with the scala version. Therefore, we can remove the scala version from the build parameters entirely.

Brief change log

• Removed scala-2.12 system variable from release scripts and azure workflow

Verifying this change

• The nightly deployment should still look the same (related to the tools/releasing/deploy_staging_jars.sh change)
• The binary release artifacts shouldn't have changed (related to the tools/releasing/create_binary_release.sh change)

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, 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

[flinkbot]

CI report:

• c305483 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[MartijnVisser]

LGTM % CI passing