Remove redundant bn->top computation.

One less to worry about.

Bug: 232
Change-Id: Ib7d38e18fee02590088d76363e17f774cfefa59b
Reviewed-on: https://boringssl-review.googlesource.com/25252
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/fipsmodule/bn/add.c b/crypto/fipsmodule/bn/add.c
index 201c526..645e647 100644
--- a/crypto/fipsmodule/bn/add.c
+++ b/crypto/fipsmodule/bn/add.c
@@ -199,7 +199,6 @@
 }
 
 int BN_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b) {
-  int max;
   int add = 0, neg = 0;
   const BIGNUM *tmp;
 
@@ -232,13 +231,6 @@
     return 1;
   }
 
-  // We are actually doing a - b :-)
-
-  max = (a->top > b->top) ? a->top : b->top;
-  if (!bn_wexpand(r, max)) {
-    return 0;
-  }
-
   if (BN_ucmp(a, b) < 0) {
     if (!BN_usub(r, b, a)) {
       return 0;