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.

htlcswitch/link_test.go

@@ -1908,7 +1908,6 @@ func TestChannelLinkBandwidthConsistency(t *testing.T) {
 	// We put Alice into hodl.ExitSettle mode, such that she won't settle
 	// incoming HTLCs automatically.
 	coreLink.cfg.HodlMask = hodl.MaskFromFlags(hodl.ExitSettle)
-	coreLink.cfg.DebugHTLC = true
 
 	estimator := lnwallet.NewStaticFeeEstimator(6000, 0)
 	feePerKw, err := estimator.EstimateFeePerKW(1)
@@ -2857,7 +2856,6 @@ func TestChannelLinkTrimCircuitsNoCommit(t *testing.T) {
 	// outgoing ADDs are opened, but the changes are not committed in the
 	// channel state.
 	alice.coreLink.cfg.HodlMask = hodl.Commit.Mask()
-	alice.coreLink.cfg.DebugHTLC = true
 
 	// Compute the static fees that will be used to determine the
 	// correctness of Alice's bandwidth when forwarding HTLCs.
@@ -4233,7 +4231,6 @@ func (h *persistentLinkHarness) restartLink(
 		MaxFeeUpdateTimeout: 40 * time.Minute,
 		// Set any hodl flags requested for the new link.
 		HodlMask:              hodl.MaskFromFlags(hodlFlags...),
-		DebugHTLC:             len(hodlFlags) > 0,
 		MaxOutgoingCltvExpiry: DefaultMaxOutgoingCltvExpiry,
 	}
 
@@ -4934,7 +4931,6 @@ func TestChannelLinkCleanupSpuriousResponses(t *testing.T) {
 	// Settle Alice in hodl ExitSettle mode so that she won't respond
 	// immediately to the htlc's meant for her. This allows us to control
 	// the responses she gives back to Bob.
-	coreLink.cfg.DebugHTLC = true
 	coreLink.cfg.HodlMask = hodl.ExitSettle.Mask()
 
 	// Add two HTLCs to Alice's registry, that Bob can pay.