Cygwin: FIFO: remove the arm method

There's no reason to check for errors when we set read_ready or
write_ready.  We don't do that for other events.

winsup/cygwin/fhandler.h

@@ -1343,7 +1343,6 @@ public:
   void set_close_on_exec (bool val);
   void __reg3 raw_read (void *ptr, size_t& ulen);
   ssize_t __reg3 raw_write (const void *ptr, size_t ulen);
-  bool arm (HANDLE h);
   bool need_fixup_before () const { return reader; }
   int fixup_before_fork_exec (DWORD) { stop_listen_client (); return 0; }
   void init_fixup_before ();