enableMajorityReadConcern option cannot be disabled if specified

    if (params.count("replication.enableMajorityReadConcern")) {

        replSettings.setMajorityReadConcernEnabled(true);

    }

This code should be using the value of the parameter to call setMajorityReadConcernEnabled, not a hardcoded "true".
Right now, if you specify {replication

} in a config file, it actually turns the option on.