Remove dependency of string_view on iostreams

Author: chris-durand

include/string_view

@@ -37,11 +37,9 @@
 
 #if __cplusplus >= 201703L
 
-#include <iosfwd>
 #include <bits/char_traits.h>
 #include <bits/functional_hash.h>
 #include <bits/range_access.h>
-#include <bits/ostream_insert.h>
 #include <ext/numeric_traits.h>
 
 namespace std _GLIBCXX_VISIBILITY(default)
@@ -613,14 +611,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return __x.compare(__y) >= 0; }
 #endif // three-way comparison
 
-  // [string.view.io], Inserters and extractors
-  template<typename _CharT, typename _Traits>
-    inline basic_ostream<_CharT, _Traits>&
-    operator<<(basic_ostream<_CharT, _Traits>& __os,
-	       basic_string_view<_CharT,_Traits> __str)
-    { return __ostream_insert(__os, __str.data(), __str.size()); }
-
-
   // basic_string_view typedef names
 
   using string_view = basic_string_view<char>;