btrfs: remove unused key type set/get helpers

The switch to open coded set/get has happend long time ago in
962a298f

 ("btrfs: kill the key type accessor helpers"), remove the
stray helpers.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>