btrfs: btrfs_defrag_root() doesn't defrag extent root tree

Since btrfs_defrag_leaves() does not support extent_root, remove its
corresponding call. The user can use the file based defrag to defrag
extents as of now.

No change in behaviour as extent_root is explicitly skipped in
btrfs_defrag_leaves and this has never worked as expected.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ ehnance changelong ]
Signed-off-by: David Sterba <dsterba@suse.com>