Remove render_dashboard_gold_trial feature flag
What does this MR do?
We added this flag in 1a9fda7a / !29014 (merged) to be cautious, as it was a change in behaviour as a bug fix. We've never toggled it, so we can just delete it: https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues?scope=all&utf8=%E2%9C%93&state=closed&search=render_dashboard_gold_trial
I got pinged about this in #251080 (closed). See also #213820.