[FLINK-35977][doc] Missing an import in datastream.md

[guluo2016]

What is the purpose of the change

This PR fix FLINK-35977 that misssing an import about OpenContext.

Brief change log

• Adding an import about OpenContext in doc datastream.md
• Removing an import about Configuration in doc datastream.md

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:

• e2421af Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[snuyanzin]

Thanks for the contribution

can you please also update process_function.md and sourceSincks.md, probably data_stream_api.md as well since they have the same issue.
May be something else, need to search through the documentation to double check