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

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>