tst_QFuture: fix UB (call of member function on wrong object)

By the time QObject::destroyed() is emitted from ~QObject(), sender no
longer is-a SenderObject, only a QObject, so calling a SenderObject
member function on it is UB.

Says UBSan:

    tst_qfuture.cpp:3854:84: runtime error: member call on address 0x60200000e550 which does not point to an object of type 'SenderObject'
     0x60200000e550: note: object is of type 'QObject'
      00 00 00 00  e8 3f 96 c9 51 7f 00 00  80 3e 00 00 c0 60 00 00  02 11 00 00 08 00 00 00  16 00 00 72
                   ^~~~~~~~~~~~~~~~~~~~~~~
                   vptr for 'QObject'

Fix by removing the QObject::connect().

This, of course, breaks the test's WHEN, but I don't see how to keep
that WHEN without the UB. At least the THEN part is not invalidated,
and there doesn't appear to be another test that tests that destroying
objects before signal emission results in a cancelled future.

Amends 612f6999c81a500a024f128bdf739342d659754a.

Change-Id: I38ca4611c071e8fd200393b600210e36d4030bc6
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 4b0261fed08dc766a7eeeb1e41121f8634e9b8cb)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/corelib/thread/qfuture/tst_qfuture.cpp

@@ -3850,8 +3850,6 @@ void tst_QFuture::signalConnect()
         QSignalSpy spy(sender, &QObject::destroyed);
         sender->deleteLater();
 
-        // emit the signal when sender is being destroyed
-        QObject::connect(sender, &QObject::destroyed, [sender] { sender->emitIntArg(42); });
         spy.wait();
 
         QVERIFY(future.isCanceled());