remove greater then one attribute vals limit when creating product variants

when add a product attribute it has only one attribute value. then create_variant_id function skip that attribute, are not create attribute value in variant product's attribute_values.

***Impacted versions:***

 -master

***Steps to reproduce:***

 1. create new product template
 2. add a attribute it only has one value into variants page
 3. save it

***Current behavior:***

 - that only one attribute value not display in product variant

***Expected behavior:***

 - display all attributes in every product variant