Btrfs: Update TODO

Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/TODO

@@ -4,7 +4,6 @@
 * make a real mkfs and superblock
 * Do checksumming
 * Define FS objects in terms of different item types
-* add inode tree
 * Add block mapping tree (simple dm layer)
 * Add simple tree locking (semaphore per tree)
 * Make allocator smarter