Nullify merge_request_metrics pipeline_id on pipeline deletion

Before, when a pipeline is deleted, the associated
`merge_request_metrics` record will be deleted as well. This is a
destructive thing as `merge_request_metrics` include other data
not related to a pipeline.

To fix this issue, instead of deleting the association record, we
nullify the `pipeline_id` on `merge_request_metrics` instead. When
this happens, related build data (`latest_build_started_at` and
`latest_build_finished_at`) will be nullified as well. This is to
ensure that when those data are requested, it'll still behave the
same way as before.

Changelog: fixed