Outdated TLS configuration limit on shared tier limitations page

https://docs.atlas.mongodb.com/reference/free-shared-limitations/

Modifying CA (via "Configure TLS Certificate Root") was a temporary feature we provided to help customers transfer from their old CA to a new one. Since all customers have converted now, we don't need the function anymore. 

Scope of Changes

Remove the "Configure TLS Certificate Root" row since we have finished converting the certificate and removed the section from UI and API.