Remove wrong definition of :option_values_attributes for variants

There's not a nested attributes definition for option values in
variants. On top of that, having `:option_values_attributes` declared as
a plain Symbol would have 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

api/openapi/solidus-api.oas.yml

@@ -7185,10 +7185,6 @@ components:
           type: array
           items:
             type: integer
-        option_values_attributes:
-          type: array
-          items:
-            $ref: '#/components/schemas/option-value-input'
         options:
           type: object
           description: '`Name` will be the name/presentation of the option type and `Value` will be the name/presentation of the option value. They will be created if not exist.'

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]
     ]