Remove GC metrics from performance bar
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31331 disabled the GC metrics in the performance bar in non-development modes. This actually broke the performance bar in a few ways, including in production, and in the below-linked QA run. We should just remove this entirely for now as we already have https://gitlab.com/gitlab-org/gitlab-ce/issues/65455 open to re-implement it.
Closes https://gitlab.com/gitlab-org/quality/nightly/issues/122.