multi: Add a channel.db migration.

The new migration removes the sweeper-last-tx top level bucket
from the channel.db database.

channeldb/migration31/migration.go

@@ -0,0 +1,23 @@
+package migration31
+
+import (
+	"errors"
+
+	"github.com/btcsuite/btcwallet/walletdb"
+	"github.com/lightningnetwork/lnd/kvdb"
+)
+
+// DeleteLastPublishedTxTLB deletes the top level bucket with the key
+// "sweeper-last-tx".
+func DeleteLastPublishedTxTLB(tx kvdb.RwTx) error {
+	log.Infof("Deleting top-level bucket: %x ...", lastTxBucketKey)
+
+	err := tx.DeleteTopLevelBucket(lastTxBucketKey)
+	if err != nil && !errors.Is(err, walletdb.ErrBucketNotFound) {
+		return err
+	}
+
+	log.Infof("Deleted top-level bucket: %x", lastTxBucketKey)
+
+	return nil
+}