multi: remove debug invoices

Debug invoices are rarely used nowadays, but keep asking for maintenance
every time refactoring in primarily the invoice registry occurs. We have
passed the cost/benefit tipping point, so therefore the debug invoice
concept is removed in this commit.

Previously the debughtlc flag also controlled whether hodl masks were
active. It is safe to remove that additional condition because the hodl
masks are still guarded by the dev build tag.

peer.go

@@ -560,7 +560,6 @@ func (p *peer) addLink(chanPoint *wire.OutPoint,
 		DecodeHopIterators:     p.server.sphinx.DecodeHopIterators,
 		ExtractErrorEncrypter:  p.server.sphinx.ExtractErrorEncrypter,
 		FetchLastChannelUpdate: p.server.fetchLastChanUpdate(),
-		DebugHTLC:              cfg.DebugHTLC,
 		HodlMask:               cfg.Hodl.Mask(),
 		Registry:               p.server.invoices,
 		Switch:                 p.server.htlcSwitch,