Cygwin: pty: Stop counting number of slaves attached to pseudo console.

- The number of slaves attached to pseudo console is used only for
  triggering redraw screen. Counting was not only needless, but also
  did not work as expected. This patch removes the code for counting.

winsup/cygwin/tty.cc

@@ -242,9 +242,8 @@ tty::init ()
   screen_alternated = false;
   mask_switch_to_pcon_in = false;
   pcon_pid = 0;
-  num_pcon_attached_slaves = 0;
   term_code_page = 0;
-  need_redraw_screen = false;
+  need_redraw_screen = true;
   fwd_done = NULL;
   pcon_last_time = 0;
   pcon_in_empty = true;