Pipeline stream connections are not deleted when deleting a pipeline

When deleting a pipeline, all dependent data should also be deleted. In particular for collection pipeline_processor_pipelines_streams:

• delete the pipeline ID from all pipeline_ids lists
• delete the entire document, if the deleted pipeline ID was the only one in that connection

Caveat: never delete the default stream connection of the default routing pipeline "All Messages Routing". See Graylog2/graylog-plugin-enterprise/issues/9589 for context.

Current Behavior

Obsolete connection documents remain in the DB.

Steps to Reproduce (for bugs)

1. Create a pipeline and connect it to a stream
2. Delete the pipeline
3. Observe that the pipeline ID is still part of a connection document

Your Environment

• Graylog Version: 6.2