Test for blocking queue should not test for non-guaranteed condition

There is a test for the utility class BlockingQueue in mongodb/src/mongo/dbtests/basictests.cpp which tests to make sure a pop on an empty queue doesn't take too long. However, the class provides no such guarantee; under load, the pop may take an arbitrary amount of time. We should remove that check.

(this is on replication because the only user of this class is the OplogBufferBlockingQueue)