Remove the deprecated include_seconds argument from distance_of_time_in_words

Pass in an :include_seconds hash option to use this feature.
Remove deprecated constants autoload

These constants were removed in 4b97ce5eb16cc20207516387fba98bf577e2e281,
but I forgot to remove the autoload calls :sparkles:.
Remove deprecated Rails application fallback for integration testing

Set ActionDispatch.test_app instead.