Fix test

Related https://semaphoreapp.com/gitlabhq/gitlabhq/branches/pull-request-8580/builds/9
Asana service doesn't check if restrict_to_branch is defined since it can be undefined

spec/models/asana_service_spec.rb

@@ -27,7 +27,6 @@ describe AsanaService, models: true do
       end
 
       it { should validate_presence_of :api_key }
-      it { should validate_presence_of :restrict_to_branch }
     end
   end