QVector: remove brittle c2m() function

The c2m() function converts a const_iterator into an iterator, but
is a broken concept for an implicitly shared container such as
QVector, because the act of calling begin() as the starting point
already detaches and invalidates the c2m argument.

This could be fixed in c2m, but it's too easy for a caller to
cause iterator invalidation even before calling c2m, as QTBUG-44592
showed, so remove the function completely.

Task-number: QTBUG-44592
Change-Id: Ibde8cd1d78a412cc9fb621f743e52f74291aff4f
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qvector.h

@@ -265,7 +265,6 @@ private:
     {
         return (i <= d->end()) && (d->begin() <= i);
     }
-    iterator c2m(const_iterator it) { return begin() + (it - cbegin()); }
     class AlignmentDummy { Data header; T array[1]; };
 };