Remove sku in product update action when setAttributeInAllVariants is used

### Description
When there's `setAttributeInAllVariants` action, there's no need to add sku to the update action. https://docs.commercetools.com/http-api-projects-products#set-attribute-in-all-variants

### Possible Solution
Use `delete action.sku` to remove sku. Check if this is the only place where sku should be removed.
https://github.com/sphereio/sphere-node-sdk/blob/master/src/coffee/sync/utils/product.coffee#L545