Remove check for __STDC_ISO_10646__

On most systems, even if __STDC_ISO_10646__ is not declared, wchar_t
will be correct in a UTF-8 locale. The client and server both enforce
a locale check on startup.

src/terminal/parser.h

@@ -31,10 +31,6 @@
 #include "parserstate.h"
 #include "parserstatefamily.h"
 
-#ifndef __STDC_ISO_10646__
-#error "Must have __STDC_ISO_10646__"
-#endif
-
 namespace Parser {
   static const StateFamily family;