drbd: Remove pointless check

In drbd-8.4 there is always a single connection per resource,
and there is always exactly one peer_device for a device.
peer_device can not be NULL here.

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