channel+cnct: remove preimage from channel and resolution

Now that the success resolver preimage field is always populated by the
incoming contest resolver, preimage lookups earlier in the
process (channel and channel arbitrator) can mostly be removed.

witness_beacon.go

@@ -7,7 +7,6 @@ import (
 	"github.com/lightningnetwork/lnd/contractcourt"
 	"github.com/lightningnetwork/lnd/invoices"
 	"github.com/lightningnetwork/lnd/lntypes"
-	"github.com/lightningnetwork/lnd/lnwallet"
 )
 
 // preimageSubscriber reprints an active subscription to be notified once the
@@ -144,4 +143,3 @@ func (p *preimageBeacon) AddPreimages(preimages ...lntypes.Preimage) error {
 }
 
 var _ contractcourt.WitnessBeacon = (*preimageBeacon)(nil)
-var _ lnwallet.PreimageCache = (*preimageBeacon)(nil)