Cygwin: pty: Fix screen distortion after less for native apps again.

- Commit c4b060e3fe seems to be not
  enough. Moreover, it does not work as expected at all in Win10
  1809. This patch essentially reverts that commit and add another
  fix. After all, the cause of the problem was a race issue in
  switch_to_pcon_out flag. That is, this flag is set when native
  app starts, however, it is delayed by wait_pcon_fwd(). Since the
  flag is not set yet when less starts, the data which should go
  into the output_handle accidentally goes into output_handle_cyg.
  This patch fixes the problem more essentially for the cause of
  the problem than previous one.

winsup/cygwin/fhandler.h

@@ -2354,7 +2354,6 @@ class fhandler_pty_slave: public fhandler_pty_common
   void setup_locale (void);
   void set_freeconsole_on_close (bool val);
   void trigger_redraw_screen (void);
-  void wait_pcon_fwd (void);
   void pull_pcon_input (void);
   void update_pcon_input_state (bool need_lock);
 };