Remove unnecessary forward declaration in std namespace

qiterator.h doesn't use std::bidirectional_iterator_tag or
std::random_access_iterator_tag, so remove the forward declarations.
(These actually confuse clang when building with C++11 support enabled).

Change-Id: Idd0daa0840a0995e6e5b0ffc01ddcdf06f048149
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qiterator.h

@@ -46,14 +46,8 @@
 
 QT_BEGIN_HEADER
 
-namespace std {
-    struct bidirectional_iterator_tag;
-    struct random_access_iterator_tag;
-}
-
 QT_BEGIN_NAMESPACE
 
-
 #define Q_DECLARE_SEQUENTIAL_ITERATOR(C) \
 \
 template <class T> \