crypto: xts - remove dependency on EXPERIMENTAL

XTS has been EXPERIMENTAL since it was introduced in 2007. I'd say by now
it has seen enough testing to justify removal of EXPERIMENTAL tag.

CC: Rik Snel <rsnel@cube.dyndns.org>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>