btrfs: scrub: remove unused path inside scrub_stripe()

The variable @path is no longer passed into any call sites after commit
18d30ab9 ("btrfs: scrub: use scrub_simple_mirror() to handle RAID56
data stripe scrub"), thus we can remove the variable completely.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>