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

lib/mpi/mpi-pow.c

@@ -41,7 +41,7 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod)
 	mpi_ptr_t tspace = NULL;
 	mpi_ptr_t rp, ep, mp, bp;
 	mpi_size_t esize, msize, bsize, rsize;
-	int esign, msign, bsign, rsign;
+	int msign, bsign, rsign;
 	mpi_size_t size;
 	int mod_shift_cnt;
 	int negative_result;
@@ -53,7 +53,6 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod)
 	esize = exp->nlimbs;
 	msize = mod->nlimbs;
 	size = 2 * msize;
-	esign = exp->sign;
 	msign = mod->sign;
 
 	rp = res->d;