Simplify a bit from pipelines_table_row.vue

The following discussion from gitlab-ce!21393 should be addressed:

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

  I know this is unrelated to the MR, but this whole block is a bit strange. I think it could be reduced to the following without running afoul of eslint:

  const ref = { ...this.pipeline.ref };
  if (ref.path) {
    ref.ref_url = ref.path;
    delete ref.ref_url;
  }
  return ref;