As of SERVER-96340, we've removed `PriorityTicketHolder` from the codebase. As part of follow-on cleanup efforts, we should also remove any code that marks internal operations or certain other operations (eg. full index scans) as low priority. This designation has no functional impact and should be removed to reduce confusion. Example here.

Also clean up any lingering templated usages of TicketHolder since it's no longer virtualized.