[FLINK-11311] [datastream] fix StreamSource implements StreamOperator interface multiple times

[libenchao]

What is the purpose of the change

https://issues.apache.org/jira/browse/FLINK-11311
This pull request fix the bug that StreamSource implements StreamOperator interface multiple times, which is unnecessary.

Brief change log

• The StreamSource implements StreamOperator just once.

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)

[StefanRRichter]

LGTM 👍