Remove workaround to build namespace owner inline

This prevents having to modify the owner after it has been frozen from a
create_default

spec/factories/namespaces.rb

@@ -5,22 +5,7 @@ FactoryBot.define do
     sequence(:name) { |n| "namespace#{n}" }
     path { name.downcase.gsub(/\s/, '_') }
 
-    # This is a workaround to avoid the user creating another namespace via
-    # User#ensure_namespace_correct. We should try to remove it and then
-    # we could remove this workaround
-    association :owner, factory: :user, strategy: :build
-    before(:create) do |namespace|
-      owner = namespace.owner
-
-      if owner
-        # We're changing the username here because we want to keep our path,
-        # and User#ensure_namespace_correct would change the path based on
-        # username, so we're forced to do this otherwise we'll need to change
-        # a lot of existing tests.
-        owner.username = namespace.path
-        owner.namespace = namespace
-      end
-    end
+    owner { association(:user, strategy: :build, namespace: instance, username: path) }
 
     trait :with_aggregation_schedule do
       after(:create) do |namespace|

spec/models/namespace_spec.rb

@@ -141,7 +141,7 @@ RSpec.describe Namespace do
       end
 
       it 'allows updating other attributes for existing record' do
-        namespace = build(:namespace, path: 'j')
+        namespace = build(:namespace, path: 'j', owner: create(:user))
         namespace.save(validate: false)
         namespace.reload