Remove the unused |Ni| member of |BN_MONT_CTX|.

Change-Id: I0a542c48c7adae28f05778d6c34c9b6836fc3449
Reviewed-on: https://boringssl-review.googlesource.com/6480
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/bn.h b/include/openssl/bn.h
index 01115c8..cbc1ee7 100644
--- a/include/openssl/bn.h
+++ b/include/openssl/bn.h
@@ -836,10 +836,7 @@
 struct bn_mont_ctx_st {
   BIGNUM RR; /* used to convert to montgomery form */
   BIGNUM N;  /* The modulus */
-  BIGNUM Ni; /* R*(1/R mod N) - N*Ni = 1
-              * (Ni is only stored for bignum algorithm) */
-  BN_ULONG n0[2]; /* least significant word(s) of Ni;
-                     (type changed with 0.9.9, was "BN_ULONG n0;" before) */
+  BN_ULONG n0[2]; /* least significant words of (R*Ri-1)/N */
   int ri;    /* number of bits in R */
 };