Workaround warning in QHash::unite

The warning pragmas don't work in gcc due to the fact this is a
template.

I've been told that unite() will disappear but meanwhile i think it's
better if we simply don't give a warning that people can't protect
themselves against

Change-Id: I358e629be86e0e675ef3e49a7fbc4f7f65ae97f6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>

src/corelib/tools/qhash.h

@@ -616,10 +616,7 @@ Q_INLINE_TEMPLATE QHash<Key, T> &QHash<Key, T>::unite(const QHash &other)
         QHash copy(other);
         const_iterator it = copy.constEnd();
         while (it != copy.constBegin()) {
-            QT_WARNING_PUSH
-            QT_WARNING_DISABLE_DEPRECATED
-            --it;
-            QT_WARNING_POP
+            it.i = QHashData::previousNode(it.i);
             insertMulti(it.key(), it.value());
         }
 #else