Fix bug where pipeline can't be removed from other pipelines if not present

This issue has been found at: https://ligo-caltech.sentry.io/issues/4107268120/?query=is%3Aunresolved&referrer=issue-stream&statsPeriod=14d&stream_index=0

This error occurs here: https://git.ligo.org/emfollow/ligo-followup-advocate/-/blob/b4ad383065742f54af54b462f23c32d2621f68a3/ligo/followup_advocate/init.py#L47

This seems like a bug elsewhere where the preferred event's pipeline hasn't be added as a pipeline preferred event, but it may make more sense to change the code back to the original design which didn't have these issues.