Remove handling of duplicate storage diffs in watcher

- Can push this responsibility down to the transformers
- Update docs to reflect that transformers should handle duplicates

libraries/shared/storage/utils/errors.go

@@ -17,12 +17,9 @@
 package utils
 
 import (
-	"errors"
 	"fmt"
 )
 
-var ErrRowExists = errors.New("parsed row for storage diff already exists")
-
 type ErrContractNotFound struct {
 	Contract string
 }