Don't quit zenity when the input stream is closed

When using --listen for a notification, zenity quits when the stream
ends. This makes it impossible to read commands from a pipe as in
`echo icon:info | zenity --notification --listen'.

https://bugzilla.gnome.org/show_bug.cgi?id=525596

src/notification.c

@@ -295,8 +295,6 @@ zenity_notification_handle_stdin (GIOChannel *channel,
 
   if ((condition & G_IO_HUP) != 0) {
     g_io_channel_shutdown (channel, TRUE, NULL);
-    zen_data->exit_code = zenity_util_return_exit_code (ZENITY_OK);
-    gtk_main_quit ();
     return FALSE;
   }