Fix header card factory

It was using methods which don't exist.

spec/factories/administration.rb

@@ -79,9 +79,6 @@ FactoryBot.define do
 
     trait :header do
       header { true }
-      sequence(:button_text)   { |n| "Button text #{n}" }
-      sequence(:button_url)    { |n| "Button url #{n}" }
-      alignment { "background" }
     end
 
     after :create do |widget_card|

spec/models/widget/card_spec.rb

@@ -10,6 +10,10 @@ describe Widget::Card do
       expect(card).to be_valid
     end
 
+    it "is valid with a header" do
+      expect(build(:widget_card, :header)).to be_valid
+    end
+
     it "is not valid without a title" do
       expect(build(:widget_card, title: "")).not_to be_valid
     end