drbd: Remove obsolete check

Smatch complained about it this redundanct check.

The check was introduced in 2006-09-13. On 2007-07-24 the body of the
function was enclosed by get_ldev()/put_ldev() reference counting.
Since then the check is useless and miss leading.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>