headers: Drop _G_config.h from public headers.

Adjust the two legacy stdc++ headers that still included it;
it seems they didn't themselves depend on any of the definitions it
contained. However, streambuf.h actually did, so put that definition in
libio.h so that it's always present.

(The stdc++ implementation will get the 'real' _G_config.h from
libroot glibc.)

headers/cpp/streambuf.h

@@ -55,12 +55,8 @@ extern "C" {
 #endif
 
 #ifndef _IO_wchar_t
-#if _G_IO_IO_FILE_VERSION == 0x20001
-#define _IO_wchar_t _G_wchar_t
-#else
 #define _IO_wchar_t wchar_t
 #endif
-#endif
 
 extern "C++" {
 class istream; /* Work-around for a g++ name mangling bug. Fixed in 2.6. */