drbd: Remove some useless paranoia code

The open_cnt check is an open-coded D_ASSERT() check.

In case the data.work queue is not empty, it does not really help to
know which drbd_work elements remained on that list: they will be freed
immediately afterwards, anyway.

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