md/raid1: remove unused bio in sync_request_write

The "bio" is not used in sync_request_write after commit a68e5870


("md/raid1: split out two sub-functions from sync_request_write").

Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>