Remove sentry metric from release manager dashboard

What does this MR do?

Removes the sentry metric from the release manager dashboard

The sentry metric reports the number of sentry issues per environment, while informative the numbers are not actionable from the release manager side. Furthermore, both release managers and the production health system don't rely on sentry issues to decide if a deployment should continue or not.

This commit removes this metric in order to make this dashboard more actionable for the release managers and the Delivery team.

Snapshot: https://dashboards.gitlab.net/dashboard/snapshot/g6h7GI58FqPdXmdwuQNkMw3gRFa0TIod?orgId=1