Remove unused condition variable in Session

While investigating BFs we noticed that the condition variable Session::_commitcv is notified but never waited on. Apparently it was added by accident.

https://github.com/mongodb/mongo/blob/master/src/mongo/db/session.cpp#L1014

https://github.com/mongodb/mongo/blob/ecf8ad987548705e773d23d5ddc3973cbc1ea7e5/src/mongo/db/session.h#L455