Update/remove remaining references to KeysInterface

[tnull]

See #226 (comment)

[tnull]

@ConorOkus I don't think this is completed, the occurances of KeysInterface the above comment mentions are still present in current main? For example:

lightningdevkit.org/docs/key_management.md

Line 189 in 300e801

If you are not using `KeysManager` for keys generation, you must re-derive the private keys yourself. Any `ChannelSigner` object must provide a unique id via the `channel_keys_id` function, whose value is provided back to you in the `SpendableOutputs` objects. A `SpendableOutputDescriptor::StaticOutput` element does not have this information as the output is sent to an output which used only `KeysInterface` data, not per-channel data.

and the .as_KeysInterface() occurrences in the Java examples of docs/tutorials/build_a_node_in_java.md and docs/payments/receiving_payments.md.

[ConorOkus]

Ahh thanks, I can get to these next week.