crypto: x86/curve25519 - Remove unused carry variables

The carry variables are assigned but never used, which upsets
the compiler.  This patch removes them.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>