Pipeline connections are not / can not be deleted via API

## Expected Behavior
Pipeline connections are deleted/removed after deleting a pipeline or stream

## Current Behavior

After deleting a stream or pipeline, Entries remain in the connections
Attempting to modify a connection through the API results in "Stream <> not found!" or "No pipeline with id " responses.

## Possible Solution

a DELETE endpoint `/system/pipelines/connections/` to be able to clear out an empty stream ID
When a pipeline is deleted, the pipeline is cleared from all stream connections
When a stream is deleted, it is removed from the connections

## Steps to Reproduce (for bugs)

1. Create a Stream
2. Create a pipeline
3. Connect stream to pipeline
4. Delete Stream, or Pipeline
5. Response data from `GET /system/pipelines/connections` still contains data for deleted stream/pipeline

## Context

I was playing around with the API, and noticed this behavior when trying to write a script to automate some tasks, that connections stick around.

## Your Environment

* Graylog Version: 5.2.6/6.0.4
* Java Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: