TTY: pty, cleanup the pty counting

Instead of the hackish way of counting ptys, let's define a specific
->remove hook both from slave and master. And decrease the count only
for master.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>