Avoid unwrap'ing `channel_parameters` in to_counterparty signing

Avoid unwrap'ing `channel_parameters` in to_counterparty signing

Previously, `StaticPaymentOutputDescriptor`s did not include
`channel_parameters` for the signer. As a result, when going to
spend old `StaticPaymentOutputDescriptor`s,
`InMemorySigner::sign_counterparty_payment_input` may be called
with `channel_parameters` set to `None`. This should be fine, but
in fa2a2efef47b5da48ac7a239f3d8a835a7f28164 we started relying on
it (indirectly via `channel_features`) for signing. This caused an
`unwrap` when spending old output descriptors.

This is fixed here by simply avoiding the unwrap and assuming old
`StaticPaymentOutputDescriptor`s represent non-anchor channels.