remove old example from migrations package

migrations/migrations.go

@@ -27,31 +27,8 @@ import (
 //		+ SyncStageProgress = []byte("SSP2")
 // - in the beginning of migration: check that old bucket exists, clear new bucket
 // - in the end:drop old bucket (not in defer!).
-//	Example:
-//	Up: func(db ethdb.Database, tmpdir string, OnLoadCommit etl.LoadCommitHandler) error {
-//		if exists, err := db.(ethdb.BucketsMigrator).BucketExists(dbutils.SyncStageProgressOld1); err != nil {
-//			return err
-//		} else if !exists {
-//			return OnLoadCommit(db, nil, true)
-//		}
-//
-//		if err := db.(ethdb.BucketsMigrator).ClearBuckets(dbutils.SyncStageProgress); err != nil {
-//			return err
-//		}
-//
-//		extractFunc := func(k []byte, v []byte, next etl.ExtractNextFunc) error {
-//			... // migration logic
-//		}
-//		if err := etl.Transform(...); err != nil {
-//			return err
-//		}
-//
-//		if err := db.(ethdb.BucketsMigrator).DropBuckets(dbutils.SyncStageProgressOld1); err != nil {  // clear old bucket
-//			return err
-//		}
-//	},
 // - if you need migrate multiple buckets - create separate migration for each bucket
-// - write test where apply migration twice
+// - write test - and check that it's safe to apply same migration twice
 var migrations = map[kv.Label][]Migration{
 	kv.ChainDB: {
 		headerPrefixToSeparateBuckets,