[libc++] Remove some _LIBCPP_CXX03_LANG from iostreams headers.

With the STL containers, I didn't enable move operations in C++03 mode
because that would change the overload resolution for things that today
are copy operations. With iostreams, though, the copy operations aren't
present at all, and so I see no problem with enabling move operations
even in (Clang's greatly extended) C++03 mode.

Clang's C++03 mode does not support delegating constructors.

Differential Revision: https://reviews.llvm.org/D104310

libcxx/test/std/input.output/string.streams/stringstream.cons/stringstream.assign/move.pass.cpp

@@ -6,8 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03
-
 // <sstream>
 
 // template <class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> >