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/raid5.c b/drivers/md/raid5.c
index cd2f96b..022a0d9 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5050,8 +5050,7 @@
 	return reshape_sectors;
 }
 
-/* FIXME go_faster isn't used */
-static inline sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int *skipped, int go_faster)
+static inline sector_t sync_request(struct mddev *mddev, sector_t sector_nr, int *skipped)
 {
 	struct r5conf *conf = mddev->private;
 	struct stripe_head *sh;