bolt11: remove payment secret workaround

[morehouse]

Previously some lightning implementations required payment secrets in all invoices while others did not. But now that https://github.com/lightning/bolts/pull/1242 has been merged to the spec, all implementations should start requiring payment secrets.

We should make sure all implementations update to comply with the latest spec change, and then remove our workaround here:

https://github.com/brunoerg/bitcoinfuzz/blob/e8ef846811381c9326f8a9b9335a112caab4d505/modules/ldk/ldk_lib/src/lib.rs#L75-L80