Remove useless Q_NO_DECLARED_NOT_DEFINED flag

The flag is used only in qstring.h and gives no real value.

Task-number: QTBUG-37437

Change-Id: I7513b56af208a5edee8452b8bbcb9b128e25133d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/corelib/tools/qstring.h

@@ -711,7 +711,7 @@ public:
     Q_DECL_CONSTEXPR inline QString(QStringDataPtr dd) : d(dd.ptr) {}
 
 private:
-#if defined(QT_NO_CAST_FROM_ASCII) && !defined(Q_NO_DECLARED_NOT_DEFINED)
+#if defined(QT_NO_CAST_FROM_ASCII)
     QString &operator+=(const char *s);
     QString &operator+=(const QByteArray &s);
     QString(const char *ch);