Fix missing translation by using AR default

Using the default fixes a missing translation. The text is slightly
changed but should be okay as well:

- "^Tax Category is required"
+ "Tax category can't be blank"

app/models/spree/product_decorator.rb

@@ -23,10 +23,9 @@ Spree::Product.class_eval do
   attr_accessible :variant_unit, :variant_unit_scale, :variant_unit_name, :unit_value
   attr_accessible :inherits_properties, :sku
 
-  # validates_presence_of :variants, unless: :new_record?, message: "Product must have at least one variant"
   validates_presence_of :supplier
   validates :primary_taxon, presence: true
-  validates :tax_category_id, presence: { message: I18n.t("validation_msg_tax") }, if: "Spree::Config.products_require_tax_category"
+  validates :tax_category_id, presence: true, if: "Spree::Config.products_require_tax_category"
 
   validates_presence_of :variant_unit
   validates_presence_of :variant_unit_scale,

config/locales/en.yml

@@ -2394,7 +2394,6 @@ See the %{link} to find out more about %{sitename}'s features and to start using
   enterprise_fee_by: "%{type} fee by %{role} %{enterprise_name}"
   validation_msg_relationship_already_established: "^That relationship is already established."
   validation_msg_at_least_one_hub: "^At least one hub must be selected"
-  validation_msg_tax: "^Tax Category is required"
   validation_msg_tax_category_cant_be_blank: "^Tax Category can't be blank"
   validation_msg_is_associated_with_an_exising_customer: "is associated with an existing customer"
   content_configuration_pricing_table: "(TODO: Pricing table)"