crypto: s390 - clean up DES code a bit more

Commit 98971f84 ("crypto: s390 - cleanup
DES code") should have also removed crypto_des.h. That file is unused
and unneeded since that commit. So let's clean up that file too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>