Remove ui_register_input_event_handler

This patch removes ui_register_input_event_handler and
ui_unregister_input_event_handler, replacing them with methods on
'ui'.  It also changes gdb to use these methods everywhere, rather
than sometimes reaching in to the ui to manage the file descriptor
directly.

gdb/infrun.c

@@ -4096,7 +4096,7 @@ check_curr_ui_sync_execution_done (void)
     {
       target_terminal::ours ();
       gdb::observers::sync_execution_done.notify ();
-      ui_register_input_event_handler (ui);
+      ui->register_file_handler ();
     }
 }