lib/mpi: Remove unused scalar_copied

The scalar_copied variable is not as the scalar is never copied
in that block.  This patch removes it.

Fixes: d58bb7e5 ("lib/mpi: Introduce ec implementation to...")
Reported-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>