During early prototyping, WriteConcernFrontierManager was originally created with the idea that it would be used to track the different frontiers from the log service. However, right now, we only support one type of write concern (majority), so this class is effectively never being used. 

We only use one of its functions to update the _committedOpTime on the LogServerManager here. and this can be replaced by directly updating that field. The underlying _frontiers data structure is never actually accessed, so for code cleanliness/clarity we should delete it in the interim.

This was determined as a part of SPM-4451 (LogServerManager Refactor), but given that its not essential to the goals of the project and just code cleanup, I'm filing a separate ticket to track this.