Drop `detached_merge_request_pipeline` and `merge_request_pipeline` keys from MR widget API

Problem

Currently, we're sending a flag in MR widget API, if the pipeline is:

• detached_merge_request_pipeline or not
• merge_request_pipeline or not

however, since we introduce new key pipeline.type in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14664, we no longer need to send these keys, and we can just use pipeline.type everywhere.