rbd: kill notify_timeout option

The "notify_timeout" rbd device option is never used, so get rid of
it.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 84d34dcc116e117a41c6fc8be13430529fc2d9e7)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>