md: remove obsolete ret in md_start_sync

The ret is not needed anymore since we have already
move resync_start into md_do_sync in commit 41a9a0dc.

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