QVariant: remove outdated docs about reimplementing clear()

QVariant::clear() hasn't been virtual since at least Qt 4...

Pick-to: 6.5 6.2 5.15
Change-Id: If6b5669d91f8d9c7ed2d152d18852a8d6f8e491a
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qvariant.cpp

@@ -498,11 +498,6 @@ void QVariant::create(QMetaType type, const void *copy)
     \fn QVariant::~QVariant()
 
     Destroys the QVariant and the contained object.
-
-    Note that subclasses that reimplement clear() should reimplement
-    the destructor to call clear(). This destructor calls clear(), but
-    because it is the destructor, QVariant::clear() is called rather
-    than a subclass's clear().
 */
 
 QVariant::~QVariant()