Revert "Implement `connections_count` metric"

This is a revert of !1359 (merged).

That change is likely one of the factors contributing to gitlab-com/gl-infra/production#17753 (closed).

Quoting @timofurrer's analysis:

My theory at the moment is that we simply have lots of connections - every agent replica has some that are auto scaled. But possibly not as much as we are seeing. Howeever, it was enough so thst the multi exec went out of bounds causing none to be removed, just making everything worse and accumulating entries in that key.

The fact that we don't have a COUNT of Hscan causes the Hscan issue with all those not- removed expired entries.