btrfs: Remove unused variables

clear_super - usage was removed in commit cea67ab9 ("btrfs: clean
the old superblocks before freeing the device") but that change forgot
to remove the actual variable.

max_key - commit 6174d3cb ("Btrfs: remove unused max_key arg from
btrfs_search_forward") removed the max_key parameter but it forgot to
remove references from callers.

stripe_len - this one was added by e06cd3dd ("Btrfs: add validadtion
checks for chunk loading") but even then it wasn't used.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>