Valgrind reveals memory management problem in QThreadPool autotest

In qtbase/tests/auto/corelib/concurrent/qthreadpool/tst_qthreadpool.cpp, the setMaxThreadCountStartsAndStopsThreads() test function ends with a commented-out delete statement, which was added as part of a seemingly unrelated commit.

Running valgrind confirms that the object that would have been deleted is leaked. If the delete is reinstated, valgrind instead complains multiple times that deleted memory is being accessed by the QSemaphore destructor, which is called twice from the WaitingTask destructor.

Rather than commenting out the delete statement, this test should be corrected to address the problems revealed by valgrind when the delete statement is executed.