qfuture_impl.h: remove redundant declaration

QtPrivate::watchContinuationImpl() is already declared in
qfutureinterface.h, which qfuture_impl.h already includes.

Fixes GCC -Wredundant-decls.

Amends 59e21a536f.

Pick-to: 6.7
Task-number: QTBUG-115583
Change-Id: I9c77209d2d10492f4d81aacc1f1da3201b5d1810
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Arno Rehn <a.rehn@menlosystems.com>

src/corelib/thread/qfuture_impl.h

@@ -588,9 +588,6 @@ void Continuation<Function, ResultType, ParentResultType>::create(F &&func,
     f->d.setContinuation(ContinuationWrapper(std::move(continuation)), fi.d);
 }
 
-// defined in qfutureinterface.cpp:
-Q_CORE_EXPORT void watchContinuationImpl(const QObject *context, QSlotObjectBase *slotObj,
-                                         QFutureInterfaceBase &fi);
 template <typename Continuation>
 void watchContinuation(const QObject *context, Continuation &&c, QFutureInterfaceBase &fi)
 {