staging/skein: Remove Skein and Threefish code

It's been four years since this was added.  In the interim, skein has
not seen any mainstream adoption.  Same with the threefish block cipher
upon which it's based.

In the discussion over which hash algorithm will replace SHA1 in git,
it's not one of the contenders.

There's absolutely no reason to think that there is anything wrong with
Skein or Threefish.  The only reason for this removal is a lack of
adoption.

If a real user comes forward, I'd be happy to assist with integrating
this code into mainline.

Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>