contractcourt+sweep: offer direct-preimage spend via SweepInput

This commit removes the method `CreateSweepTx` and makes sure when
sweeping the htlc output via the direct-preimage spend, it's offered via
the `SweepInput` interface.

contractcourt/briefcase_test.go

@@ -331,7 +331,6 @@ func TestContractInsertionRetrieval(t *testing.T) {
 		htlc: channeldb.HTLC{
 			RHash: testPreimage,
 		},
-		sweepTx: nil,
 	}
 	resolvers := []ContractResolver{
 		&timeoutResolver,