Btrfs: cleanup dead code of defragment

@is_extent is no more needed since we don't defrag extent root.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/tree-defrag.c

@@ -37,7 +37,6 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
 	int ret = 0;
 	int wret;
 	int level;
-	int is_extent = 0;
 	int next_key_ret = 0;
 	u64 last_ret = 0;
 	u64 min_trans = 0;
@@ -50,7 +49,7 @@ int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
 		goto out;
 	}
 
-	if (root->ref_cows == 0 && !is_extent)
+	if (root->ref_cows == 0)
 		goto out;
 
 	if (btrfs_test_opt(root, SSD))