QStringView: fix ambiguity between f(QString)/f(QStringView) with QString::null args

... by removing the QStringView(QString::Null) ctor.

This means that QStringView(QString::null) will now fail to compile.
That's ok, since we've deprecated QString::null in the last Qt version.

This will also help pulling up the QStringView definition to before
the QString one in qstring.h, something that we'll require for properly
adding QStringView-append/prepend/insert to QString.

Change-Id: I3c836732be1e185111b13950be9d8dcd935734ff
Reviewed-by: Anton Kudryavtsev <antkudr@mail.ru>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/tools/qstringview.h

@@ -170,10 +170,6 @@ public:
         : m_size(0), m_data(nullptr) {}
     Q_DECL_CONSTEXPR QStringView(std::nullptr_t) Q_DECL_NOTHROW
         : QStringView() {}
-#if QT_DEPRECATED_SINCE(5, 9)
-    Q_DECL_CONSTEXPR QStringView(QString::Null) Q_DECL_NOTHROW
-        : QStringView() {}
-#endif
 
     template <typename Char, if_compatible_char<Char> = true>
     Q_DECL_CONSTEXPR QStringView(const Char *str, qssize_t len)