Remove the last remnants of key_exchange_info.

DHE ciphers are gone, so we no longer need to clear drop the "group_id"
field there. That leaves static RSA, but:

- We mass-invalidated every serialized client session in
  364f7a6d21f73d099f4f80800973174a8f71e90f, long after we stopped
  filling in key_exchange_info on the client.

- Server sessions were not mass-invalidated, but static RSA
  key_exchange_info never worked on the server.

This means it is safe to remove this logic.

Change-Id: Id43b233cca066a81686be7c056c530ba8e89f761
Reviewed-on: https://boringssl-review.googlesource.com/15005
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/ssl_asn1.c

@@ -121,9 +121,6 @@
  *                                  -- stapled OCSP response from the server
  *     extendedMasterSecret    [17] BOOLEAN OPTIONAL,
  *     groupID                 [18] INTEGER OPTIONAL,
- *                                  -- For historical reasons, for static RSA
-                                    -- ciphers, this field contains another
-                                    -- value to be discarded.
  *     certChain               [19] SEQUENCE OF Certificate OPTIONAL,
  *     ticketAgeAdd            [21] OCTET STRING OPTIONAL,
  *     isServer                [22] BOOLEAN DEFAULT TRUE,
@@ -692,24 +689,11 @@ SSL_SESSION *SSL_SESSION_parse(CBS *cbs, const SSL_X509_METHOD *x509_method,
   }
   ret->extended_master_secret = !!extended_master_secret;
 
-  uint32_t value;
-  if (!SSL_SESSION_parse_u32(&session, &value, kGroupIDTag, 0)) {
+  if (!SSL_SESSION_parse_u16(&session, &ret->group_id, kGroupIDTag, 0)) {
     OPENSSL_PUT_ERROR(SSL, SSL_R_INVALID_SSL_SESSION);
     goto err;
   }
 
-  /* Historically, the group_id field was used for key-exchange-specific
-   * information. Discard all but the group ID. */
-  if (ret->cipher->algorithm_mkey & SSL_kRSA) {
-    value = 0;
-  }
-
-  if (value > 0xffff) {
-    OPENSSL_PUT_ERROR(SSL, SSL_R_INVALID_SSL_SESSION);
-    goto err;
-  }
-  ret->group_id = (uint16_t)value;
-
   CBS cert_chain;
   CBS_init(&cert_chain, NULL, 0);
   int has_cert_chain;