crypto: xts - drop gf128mul dependency

Since the gf128mul_x_ble function used by xts.c is now defined inline
in the header file, the XTS module no longer depends on gf128mul.
Therefore, the 'select CRYPTO_GF128MUL' line can be safely removed.

Signed-off-by: Ondrej Mosnacek <omosnacek@gmail.com>
Reviewd-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>