btrfs: qgroup: remove outdated TODO comments

These comments are old, outdated and not very specific. It seems that it
doesn't help to inspire anybody to work on that.  So we remove them.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Sidong Yang <realwakka@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -25,18 +25,6 @@
 #include "sysfs.h"
 #include "tree-mod-log.h"
 
-/* TODO XXX FIXME
- *  - subvol delete -> delete when ref goes to 0? delete limits also?
- *  - reorganize keys
- *  - compressed
- *  - sync
- *  - copy also limits on subvol creation
- *  - limit
- *  - caches for ulists
- *  - performance benchmarks
- *  - check all ioctl parameters
- */
-
 /*
  * Helpers to access qgroup reservation
  *