crypto: delete built-in XTS cipher mode support

crypto: delete built-in XTS cipher mode support

The built-in AES+XTS implementation is used for the LUKS encryption
When building system emulators it is reasonable to expect that an
external crypto library is being used instead. The performance of the
builtin XTS implementation is terrible as it has no CPU acceleration
support. It is thus not worth keeping a home grown XTS implementation
for the built-in cipher backend.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>