Remove gitlab_transaction_cputime_seconds metric
What does this MR do?
This metric was added in !18584 (merged), but doesn't appear to be used anywhere (I checked https://gitlab.com/gitlab-com/runbooks), and it's not documented.
Even then, we wouldn't necessarily want to remove it, but it has very high cardinality. https://prometheus-app.gprd.gitlab.net/status shows that it's the joint-highest cardinality metric, with 269,738 series!