async_xor: Remove unused 'async_xor_val'

async_xor_val has been unused since commit
a7c224a8 ("md/raid5: convert to new xor compution interface")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>