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