[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:

• bf3571b7176b657ca97c57648887a03dc2383f72 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[guluo2016]

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

Thanks for your review, the changes have been made, please review.

[snuyanzin]

@guluo2016 can you please also create backports for other versions?