Simplify tls1_channel_id_hash.

Rather than iterate over handshake_dgsts itself, it can just call
tls1_handshake_digest.

Change-Id: Ia518da540e47e65b13367eb1af184c0885908488
Reviewed-on: https://boringssl-review.googlesource.com/5617
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/s3_clnt.c b/ssl/s3_clnt.c
index 6a3f157..ef24316 100644
--- a/ssl/s3_clnt.c
+++ b/ssl/s3_clnt.c
@@ -2210,10 +2210,8 @@
   i2o_ECPublicKey(ec_key, &derp);
 
   uint8_t digest[EVP_MAX_MD_SIZE];
-  unsigned digest_len;
-  if (!EVP_DigestInit_ex(&md_ctx, EVP_sha256(), NULL) ||
-      !tls1_channel_id_hash(&md_ctx, s) ||
-      !EVP_DigestFinal_ex(&md_ctx, digest, &digest_len)) {
+  size_t digest_len;
+  if (!tls1_channel_id_hash(s, digest, &digest_len)) {
     goto err;
   }