Remove out-of-date and misleading comment in |bn_blinding_st|.

I guess the comment "just a reference" was intended to mean that the
|mod| member is a weak reference to a |BIGNUM| owned by something else.
However, it is actually owned by the |bn_blinding_st|, as one can see
by reading |BN_BLINDING_new| and |BN_BLINDING_free|.

Change-Id: If2a681fc9d9db536170e0efb11fdab93e4f0baba
Reviewed-on: https://boringssl-review.googlesource.com/7112
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/crypto/rsa/blinding.c b/crypto/rsa/blinding.c
index d5bfcd3..e6b987d 100644
--- a/crypto/rsa/blinding.c
+++ b/crypto/rsa/blinding.c
@@ -123,7 +123,7 @@
   BIGNUM *A;
   BIGNUM *Ai;
   BIGNUM *e;
-  BIGNUM *mod; /* just a reference */
+  BIGNUM *mod;
   int counter;
   /* mont is the Montgomery context used for this |BN_BLINDING|. It is not
    * owned and must outlive this structure. */