SUNRPC: Remove RPCSEC_GSS_KRB5_ENCTYPES_DES

Make it impossible to enable support for the DES or DES3 Kerberos
encryption types in SunRPC. These enctypes were deprecated by RFCs
6649 and 8429 because they are known to be insecure.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>