QStringList: use QList:.swap(int,int) in removeDuplicates instead of assignment

This is faster because it just swaps the pointers stored instead of touching
the refcounts. It's the same as qMove()ing the right-hand-side, but benefits
C++98, too.

Change-Id: I6b7e3d0e5c7eb81f88fe9069d6662335d24aa86c
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/tools/qstringlist.cpp

@@ -726,7 +726,7 @@ int QtPrivate::QStringList_removeDuplicates(QStringList *that)
             continue;
         seen.insert(s);
         if (j != i)
-            (*that)[j] = s;
+            that->swap(i, j);
         ++j;
     }
     if (n != j)