Remove "total number of active pipelines" setting from docs

What does this MR do?

The "Maximum number of active pipelines per project" setting looks to have been fully removed from GitLab in #368195 (closed) and the linked issues and MRs. Since it no longer exists at all we should remove the mention of it from the documentation for CI limits.

A customer wrote in wondering where it was and I learned the answer was "nowhere" so this should help the confusion a little!

Related issues

#368195 (closed)

(GitLab internal only link) Customer's Zendesk Ticket

Author's checklist

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