QJsonValue: implement copy-assignment via copy-and-swap

Now we have a swap() member, remove the comment.
Amends bbb440bab2.

Change-Id: I3640d544bbd0927109460d843252c8e5f0aefaf5
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/json/qjsonvalue.cpp

@@ -276,10 +276,7 @@ QJsonValue::QJsonValue(const QJsonValue &other)
 QJsonValue &QJsonValue::operator =(const QJsonValue &other)
 {
     QJsonValue copy(other);
-    // swap(copy);
-    qSwap(dbl, copy.dbl);
-    qSwap(d,   copy.d);
-    qSwap(t,   copy.t);
+    swap(copy);
     return *this;
 }