Address race condition in Requests#pipeline_creating_for_merge_request?

The following discussion from !165363 (merged) should be addressed:

• @fabiopitino started a discussion: (+2 comments)

  This could be a surprising side-effect. Can we make it explicit by having a parameter delete_if_completed: false by default?. This way the parameter can be true when reading via GraphQL but can remain false if it's ever used internally for other checks.