Fix Error 500 when deleting a pipeline via the API
GitLab EE has an admin audit log that attempts to call
the entity for metadata. Unlike a project or a group, a CI pipeline
doesn't have this, so users deleting a pipeline would always hit an Error 500.
AuditEventService isn't equipped to handle logging of the destruction of
entities such as CI pipelines. It's a project-level event that operates
on a pipeline.
Rather than pollute the database with information that doesn't quite fit in the schema, let's remove it altogether.