xmlsec: DerivedKey leftovers

[lsh123]

https://www.w3.org/TR/xmlenc-core1/#sec-DerivedKey

• ReferenceList (optional) is not implemented (ignored).
• DerivedKeyName (optional) is used only for the current key and cannot be referenced by the ds:KeyName element in ds:KeyInfo.
• Recipient (optional) is not implemented (ignored).
• The (optional) Type attribute is not implemented (ignored), the derived key type is determined by the uplevel requirements.