[FLINK-11246][table] Fix distinct AGG visibility issue

[dianfu]

What is the purpose of the change

This pull request Fixes the visibility issue of distinct in Table API

Brief change log

(for example:)

• Remove private[flink] of DistinctAggregateFunction#distinct
• Remove private[flink] of DistinctAgg#distinct

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)

[sunjincheng121]

@dianfu Thanks for the PR.
Will be merged.

[dianfu]

@sunjincheng121 Thanks a lot for the quick review and merge. :)