drbd: Remove redundant check

Opening a device only succeeds on a primary node, or when explicitly
setting the allow_oos module parameter to allow opening the device
read-only on a secondary node.  There is no other way that a request can
get into drbd_make_request(), so this code cannot trigger.

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