drbd: remove orphan _req_may_be_done() declaration

The _req_may_be_done() has been removed by
commit 6870ca6d ("drbd: factor out master_bio completion
and drbd_request destruction paths"), so remove the orphan
declaration.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Acked-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Link: https://lore.kernel.org/r/20220920015216.782190-2-cuigaosheng1@huawei.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>