default to using new onion tlvs even without feature flags

[TheBlueMatt]

Everyone is moving towards requiring TLV onions, so we should really be defaulting to them, especially when we only have route hints and no features for a node.

[ViktorTigerstrom]

I could pick this one up if no one else is working on it!

Is the general issue that we default to the BOLT 4 legacy hop_data payload format (msgs::OnionHopDataFormat::Legacy) here when hop.node_features.supports_variable_length_onion() isn't true, hence also when there are no node features?

rust-lightning/lightning/src/ln/onion_utils.rs

Line 136 in 44c0374

format: if hop.node_features.supports_variable_length_onion() {

Given the discussion in the latest LDK dev meeting, do we just want to drop creating msgs::OnionHopDataFormat::Legacy format payloads all together in this function, or is there a scenario where we'd still like to create them?