Suppress stty error on Apple Terminal

`stty` called in `Reline::ANSI.deprep` command shows the following
error message on macOS Apple Terminal, with some settings.

```
$ LANG=C irb
irb(main):001:0>
stty: 'standard input': unable to perform all requested operations
stty: 'standard input': unable to perform all requested operations
```

Author: nobu

lib/reline/ansi.rb

@@ -135,7 +135,7 @@ def self.prep
 
   def self.deprep(otio)
     int_handle = Signal.trap('INT', 'IGNORE')
-    `stty #{otio}`
+    system("stty #{otio}", err: File::NULL)
     Signal.trap('INT', int_handle)
     Signal.trap('WINCH', @@old_winch_handler) if @@old_winch_handler
   end