Follow-up to [#49819][]

Since [#49819][] resolves an issue with
`ActionDispatch::IntegrationTest#with_routing` helper support, Action
View's `test/abstract_unit.rb` file can rely on routing being reset
within the block argument.

This means that the `RoutedRackApp` class and `.build_app` method is can
be made unnecessary.

[#49819]: https://github.com/rails/rails/pull/49819