QDuplicateTracker: remove the appendTo && overload

As a requirement for unordered_set (and QSet) both "iterator" and
"const_iterator" are const, so we cannot reassign or move the key.
That means this overload is no different to the const & overload, so
we can just remove it.

Pick-to: 6.0
Change-Id: Ia14dccf7f610967649bab38161ce6d963509316b
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/tools/qduplicatetracker_p.h

@@ -101,18 +101,11 @@ public:
     }
 
     template <typename C>
-    void appendTo(C &c) const &
+    void appendTo(C &c) const
     {
         for (const auto &e : set)
             c.push_back(e);
     }
-
-    template <typename C>
-    void appendTo(C &c) &&
-    {
-        for (auto &e : set)
-            c.push_back(std::move(e));
-    }
 };
 
 QT_END_NAMESPACE