lib/mpi: remove redundant variable esign

Variable esign is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
warning: variable 'esign' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>