Fix lint issues reported by staticcheck

Used staticcheck 2022.1.2 (v0.3.2)

See: staticcheck.io

backend/cache/storage_persistent.go

@@ -456,10 +456,7 @@ func (b *Persistent) HasEntry(remote string) bool {
 
 		return fmt.Errorf("couldn't find object (%v)", remote)
 	})
-	if err == nil {
-		return true
-	}
-	return false
+	return err == nil
 }
 
 // HasChunk confirms the existence of a single chunk of an object