Remove keybox from OEMCrypto ref

Merge from Widevine repo of http://go/wvgerrit/56526

This CL removes the test keybox from OEMCrypto reference code.

Test: unit tests
Bug: 76393338 split mock into ref and testbed
Change-Id: I4bf0eb777c6851191d0ac9ccf8e2b42c55c8f6b9

libwvdrmengine/oemcrypto/ref/src/oemcrypto_engine_ref.h

@@ -58,8 +58,8 @@ class CryptoEngine {
 
   KeyboxError ValidateKeybox() { return root_of_trust_.ValidateKeybox(); }
 
-  const std::vector<uint8_t>& DeviceRootKey(bool override_to_real = false) {
-    return root_of_trust_.DeviceKey(override_to_real);
+  const std::vector<uint8_t>& DeviceRootKey() {
+    return root_of_trust_.DeviceKey();
   }
 
   const std::vector<uint8_t>& DeviceRootId() {