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_test.go

@@ -0,0 +1,48 @@
+package migration31
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/channeldb/migtest"
+	"github.com/lightningnetwork/lnd/kvdb"
+	"github.com/stretchr/testify/require"
+)
+
+var (
+	hexStr = migtest.Hex
+
+	// lastTxBefore is the "sweeper-last-tx" bucket before the migration.
+	// We fill the last-tx value with a dummy hex string because the actual
+	// value is not important when deleting the bucket.
+	lastTxBefore = map[string]interface{}{
+		"sweeper-last-tx": hexStr("0000"),
+	}
+)
+
+// TestDeleteLastPublishTxTLP asserts that the sweeper-last-tx bucket is
+// properly deleted.
+func TestDeleteLastPublishTxTLP(t *testing.T) {
+	t.Parallel()
+
+	// Prime the database with the populated sweeper-last-tx bucket.
+	before := func(tx kvdb.RwTx) error {
+		return migtest.RestoreDB(tx, lastTxBucketKey, lastTxBefore)
+	}
+
+	// After the migration, ensure that the sweeper-last-tx bucket was
+	// properly deleted.
+	after := func(tx kvdb.RwTx) error {
+		err := migtest.VerifyDB(tx, lastTxBucketKey, nil)
+		require.ErrorContains(
+			t, err,
+			fmt.Sprintf("bucket %s not found", lastTxBucketKey),
+		)
+
+		return nil
+	}
+
+	migtest.ApplyMigration(
+		t, before, after, DeleteLastPublishedTxTLB, false,
+	)
+}