Remove the `final_cltv_expiry_delta` in `RouteParameters` entirely

Remove the `final_cltv_expiry_delta` in `RouteParameters` entirely

fbc08477e8dcdd8f3f2ada8ca77388b6185febe2 purported to "move" the
`final_cltv_expiry_delta` field to `PaymentParamters` from
`RouteParameters`. However, for naive backwards-compatibility
reasons it left the existing on in place and only added a new,
redundant field in `PaymentParameters`.

It turns out there's really no reason for this - if we take a more
critical eye towards backwards compatibility we can figure out the
correct value in every `PaymentParameters` while deserializing.

We do this here - making `PaymentParameters` a `ReadableArgs`
taking a "default" `cltv_expiry_delta` when it goes to read. This
allows existing `RouteParameters` objects to pass the read
`final_cltv_expiry_delta` field in to be used if the new field
wasn't present.