Remove unused variable

Merge from Widevine repo of http://go/wvgerrit/134728

This variable was written to, but the value was never read. Newer
compilers would complain about this. This patch removes the unused
variable.

Bug: 202400919
Change-Id: I87cae291d41b18db91a4c4f8a76edb537635db2c

libwvdrmengine/oemcrypto/odk/test/odk_test_helper.cpp

@@ -482,11 +482,6 @@ void ODK_BuildMessageBuffer(ODK_CoreMessage* core_message,
       {ODK_UINT32, &(core_message->nonce_values.session_id), "session_id"},
   };
 
-  uint32_t header_size = 0;
-  for (auto& field : total_fields) {
-    header_size += ODK_FieldLength(field.type);
-  }
-
   total_fields.insert(total_fields.end(), extra_fields.begin(),
                       extra_fields.end());
   for (auto& field : total_fields) {