Remove wrong definition of :option_values_attributes for variants

Having `:option_values_attributes` declared as a plain Symbol has no
effect. Rails requires that nested attributes are explicitly declared
[1].

Variants already support `:option_value_ids` array [2] which cover the
most common use case (adding option values to an already created
variant), so there's no need to provide such a nested hierarchy.

1 - https://api.rubyonrails.org/classes/ActionController/StrongParameters.html
2 - https://github.com/solidusio/solidus/blob/07c88ddd1603699939ac0343965fa88dc2e1851a/core/lib/spree/permitted_attributes.rb#L135

core/lib/spree/permitted_attributes.rb

@@ -131,7 +131,7 @@ module PermittedAttributes
     @@variant_attributes = [
       :name, :presentation, :cost_price, :lock_version,
       :position, :track_inventory,
-      :product_id, :product, :option_values_attributes, :price,
+      :product_id, :product, :price,
       :weight, :height, :width, :depth, :sku, :cost_currency, option_value_ids: [], options: [:name, :value]
     ]