crypto: serpent - get rid of obsolete tnepres variant

It is not trivial to trace back why exactly the tnepres variant of
serpent was added ~17 years ago - Google searches come up mostly empty,
but it seems to be related with the 'kerneli' version, which was based
on an incorrect interpretation of the serpent spec.

In other words, nobody is likely to care anymore today, so let's get rid
of it.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>