Always check client last operation time when computing operationTime to append to the response

Currently, if a command has an uninitialized startOperationTime (because the client still has uninitialized opTime when the command starts running), computeOperationTime returns the replication coordinator's lastAppliedOpTime as the operationTime to append to the response for the command. This operationTime could sometimes be later than the opTime for the operation performed by the command. To prevent this, this block of code should be removed so that we always check and try to use the client last operation time when possible.