channeldb: move deprecated code to migration_09_legacy_serialization.go

And unexport deprecated code.

channeldb/legacy_serialization.go

@@ -1,6 +1,8 @@
 package channeldb
 
-import "io"
+import (
+	"io"
+)
 
 // deserializeCloseChannelSummaryV6 reads the v6 database format for
 // ChannelCloseSummary.