appletalk: Remove unneeded synchronization

The use of del_timer_sync() will make sure a timer is not rescheduled.
As such, there is no need to add external signals to kill timers. In
preparation for switching the timer callback argument to the timer
pointer, this drops the .data argument since it doesn't serve a meaningful
purpose here.

Cc: David Howells <dhowells@redhat.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>