Bignum: Remove dead code

Both variables affected by the code are overwritten before their next
read.
diff --git a/library/bignum.c b/library/bignum.c
index c9919fb..8bdf2e9 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2104,15 +2104,6 @@
         /*
          * pick a random A, 1 < A < |X| - 1
          */
-        MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &A, X->n * ciL, f_rng, p_rng ) );
-
-        if( mbedtls_mpi_cmp_mpi( &A, &W ) >= 0 )
-        {
-            j = mbedtls_mpi_bitlen( &A ) - mbedtls_mpi_bitlen( &W );
-            MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &A, j + 1 ) );
-        }
-        A.p[0] |= 3;
-
         count = 0;
         do {
             MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &A, X->n * ciL, f_rng, p_rng ) );