Drop db_load_balancing_index (database load balancing Prometheus metrics)

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15440 we exposed db_load_balancing_index as the current index that the load balancer is currently at.

However as the host list is randomized at each client, this means that the index is effectively meaningless - see discussion at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15440#note_209590667 - and we cannot plot a dashboard

Ideally we expose a metric that is useful for https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7551

Proposal

Drop the db_load_balancing_index gauge metric as it is misleading and un-used (by GitLab.com)