md: remove 'go_faster' option from ->sync_request()

This option is not well justified and testing suggests that
it hardly ever makes any difference.

The comment suggests there might be a need to wait for non-resync
activity indicated by ->nr_waiting, however raise_barrier()
already waits for all of that.

So just remove it to simplify reasoning about speed limiting.

This allows us to remove a 'FIXME' comment from raid5.c as that
never used the flag.

Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/md.h b/drivers/md/md.h
index ecdce36..4046a6c 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -506,7 +506,7 @@
 	int (*hot_add_disk) (struct mddev *mddev, struct md_rdev *rdev);
 	int (*hot_remove_disk) (struct mddev *mddev, struct md_rdev *rdev);
 	int (*spare_active) (struct mddev *mddev);
-	sector_t (*sync_request)(struct mddev *mddev, sector_t sector_nr, int *skipped, int go_faster);
+	sector_t (*sync_request)(struct mddev *mddev, sector_t sector_nr, int *skipped);
 	int (*resize) (struct mddev *mddev, sector_t sectors);
 	sector_t (*size) (struct mddev *mddev, sector_t sectors, int raid_disks);
 	int (*check_reshape) (struct mddev *mddev);