Remove extraneous query for mysql backend

Author: cschleiden

backend/mysql/mysql.go

@@ -146,11 +146,6 @@ func (b *mysqlBackend) CreateWorkflowInstance(ctx context.Context, instance *wor
 	}
 	defer tx.Rollback()
 
-	// Check for existing instance
-	if err := tx.QueryRowContext(ctx, "SELECT 1 FROM `instances` WHERE instance_id = ? LIMIT 1", instance.InstanceID).Scan(new(int)); err != sql.ErrNoRows {
-		return backend.ErrInstanceAlreadyExists
-	}
-
 	// Create workflow instance
 	if err := createInstance(ctx, tx, instance, event.Attributes.(*history.ExecutionStartedAttributes).Metadata); err != nil {
 		return err