Remove serialization code from SequencedHashMap

[coheigea]

The deserialization code in SequencedHashMap is a security risk:

SequencedHashMap.readExternal() calls in.readObject() for both keys and values without any type restriction, which is a classic gadget-chain deserialization vector. The other readExternal implementations deserialize known types via internal delegation, but are all exposed to any caller that passes an untrusted ObjectInput.

We don't use it in the API or Directory Server so let's just remove it