Remove writing into redundant cache in build-index (#155)

Since there is the cache on Nginx side, there is no point to writing
tag info into another cache after pulling from any backend.

Removed calling to writeTagToDisk in resolveFromBackend function.

build-index/tagstore/store.go

@@ -28,7 +28,6 @@ import (
 	"github.com/uber/kraken/lib/persistedretry/writeback"
 	"github.com/uber/kraken/lib/store"
 	"github.com/uber/kraken/lib/store/metadata"
-	"github.com/uber/kraken/utils/log"
 
 	"github.com/uber-go/tally"
 )
@@ -160,8 +159,5 @@ func (s *tagStore) resolveFromBackend(tag string) (core.Digest, error) {
 	if err != nil {
 		return core.Digest{}, fmt.Errorf("parse backend digest: %s", err)
 	}
-	if err := s.writeTagToDisk(tag, d); err != nil {
-		log.With("tag", tag).Errorf("Error writing tag to disk: %s", err)
-	}
 	return d, nil
 }