btrfs: remove unnecessary extent root check in btrfs_defrag_leaves

We only defrag leaves on roots that have SHAREABLE set, so we don't need
to check if we're the extent root as it doesn't have SHAREABLE set.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>