spec: Cleanup custom emoji factory

The CustomEmoji factory was creating a namespace *and* a group, that
shouldn't have. Also the creator was set to nil by default, that's also
not good.

spec/factories/custom_emoji.rb

@@ -3,9 +3,8 @@
 FactoryBot.define do
   factory :custom_emoji, class: 'CustomEmoji' do
     sequence(:name) { |n| "custom_emoji#{n}" }
-    namespace
     group
     file { 'https://gitlab.com/images/partyparrot.png' }
-    creator { namespace.owner }
+    creator factory: :user
   end
 end

spec/models/custom_emoji_spec.rb

@@ -38,7 +38,7 @@ RSpec.describe CustomEmoji do
       new_emoji = build(:custom_emoji, name: old_emoji.name, namespace: old_emoji.namespace, group: group)
 
       expect(new_emoji).not_to be_valid
-      expect(new_emoji.errors.messages).to eq(creator: ["can't be blank"], name: ["has already been taken"])
+      expect(new_emoji.errors.messages).to eq(name: ["has already been taken"])
     end
 
     it 'disallows non http and https file value' do