Simplify user factory around admin role

The original Spree user factory was adding the admin role by default.
Since we don't do that anymore, we don't need to remove it for normal
users. We also need to add it only once to admin users.

spec/factories/user_factory.rb

@@ -35,17 +35,11 @@ FactoryBot.define do
     end
 
     after(:create) do |user, proxy|
-      user.spree_roles.clear # Remove admin role
-
       user.enterprises << proxy.enterprises
     end
 
     factory :admin_user do
       spree_roles { [Spree::Role.find_or_create_by!(name: 'admin')] }
-
-      after(:create) do |user|
-        user.spree_roles << Spree::Role.find_or_create_by!(name: 'admin')
-      end
     end
 
     factory :oidc_user do