Remove unused locals and fix tests in System.ServiceModel.Syndication (#39639)

Remove unused locals and fix tests in System.ServiceModel.Syndication (#39639)

* Remove unused locals and fix tests in System.ServiceModel.Syndication

Remove unused locals in System.ServiceModel.Syndication.

While refactoring I found two places where unit tests appear to have a
copy/paste error and were asserting the wrong value.

* Add back `thirdDocument` and correct tests

* Add back `sixthDocument` and correct tests

* Add back item and correct tests

* Add back `genericFormatter` and correct test

* Add back `elementExtensions` and add assert