Fix unsafe rubocop offenses

Using

    rubocop -A

and verified they are safe.

api/app/views/spree/api/images/_image.json.jbuilder

@@ -2,6 +2,6 @@
 
 json.(image, *image_attributes)
 json.(image, :viewable_type, :viewable_id)
-Spree::Image.attachment_definitions[:attachment][:styles].each do |key, _value|
+Spree::Image.attachment_definitions[:attachment][:styles].each_key do |key|
   json.set! "#{key}_url", image.url(key)
 end

backend/app/controllers/spree/admin/payment_methods_controller.rb

@@ -28,7 +28,7 @@ def update
         invoke_callbacks(:update, :before)
 
         attributes = payment_method_params
-        attributes.each do |key, _value|
+        attributes.each_key do |key|
           if key.include?("password") && attributes[key].blank?
             attributes.delete(key)
           end

backend/app/controllers/spree/admin/products_controller.rb

@@ -68,7 +68,7 @@ def location_after_save
         if updating_variant_property_rules?
           url_params = {}
           url_params[:ovi] = []
-          params[:product][:variant_property_rules_attributes].each do |_index, param_attrs|
+          params[:product][:variant_property_rules_attributes].each_value do |param_attrs|
             url_params[:ovi] += param_attrs[:option_value_ids]
           end
           spree.admin_product_product_properties_url(@product, url_params)
@@ -133,7 +133,7 @@ def render_after_update_error
       def normalize_variant_property_rules
         return unless updating_variant_property_rules?
 
-        params[:product][:variant_property_rules_attributes].each do |_index, param_attrs|
+        params[:product][:variant_property_rules_attributes].each_value do |param_attrs|
           param_attrs[:option_value_ids] = param_attrs[:option_value_ids].split(',')
         end
       end

core/app/models/spree/stock/splitter/shipping_category.rb

@@ -24,7 +24,7 @@ def split_by_category(package)
 
         def hash_to_packages(categories)
           packages = []
-          categories.each do |_id, contents|
+          categories.each_value do |contents|
             packages << build_package(contents)
           end
           packages