Remove NO_VSNPRINTF

Affects only ancient VMS where vsnprintf wasn't available.

src/end.c

@@ -656,11 +656,7 @@ panic VA_DECL(const char *, str)
     {
         char buf[BUFSZ];
 
-#if !defined(NO_VSNPRINTF)
         (void) vsnprintf(buf, sizeof buf, str, VA_ARGS);
-#else
-        Vsprintf(buf, str, VA_ARGS);
-#endif
         raw_print(buf);
         paniclog("panic", buf);
     }