The user transaction ioctls have been removed in kernel 4.17 by commit
7a5a07a81062 ("btrfs: Remove userspace transaction ioctls"), the
definitions are not relevant and can be removed.
The numbers could be reused in the future, eg. when there are no
maintained LTS kernels older than 4.19.
Signed-off-by: David Sterba <dsterba@suse.com>