QSpan: remove a stale static constexpr `extent` from a base class

It's not needed, and might trigger -Wshadow on some compilers. Only
the public QSpan class has the `extent` static data member, everything
else uses the template argument, `E`, directly.

Amends f82cf6333e.

Pick-to: 6.6
Change-Id: If378119aff1e352d1e90854b570720444cd532a0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>

src/corelib/tools/qspan_p.h

@@ -151,8 +151,7 @@ protected:
     static constexpr qsizetype m_size = qsizetype(E);
 
     // types and constants:
-
-    static constexpr size_t extent = E;
+    // (in QSpan only)
 
     // constructors (need to be public d/t the way ctor inheriting works):
 public: