btrfs: qgroup: delete a TODO about using kmem cache to allocate structures

Generic slab works fine allocating btrfs_qgroup_extent_record
structures. It's not necessary to create a dedicated kmem cache that
would be created but unused if quotas were not enabled. Let's delete the
TODO line.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Junchao Sun <sunjunchao2870@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.h

@@ -123,7 +123,6 @@ struct btrfs_inode;
 
 /*
  * Record a dirty extent, and info qgroup to update quota on it
- * TODO: Use kmem cache to alloc it.
  */
 struct btrfs_qgroup_extent_record {
 	struct rb_node node;