xen-blkfront: Remove redundant assignment to variable err

The variable err is being assigned a value that is never read, the
assignment is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20210806110601.11386-1-colin.king@canonical.com


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