Remove duplication in include test

This is following #11492

This code duplication goes back to 4 years ago, in 783deae99a4850f597135146b19e7ee4622da94e.
Back then, `ActiveSupport::Concern` had a `#use` method, which was removed later on in the profit of the currently used native `#include`.

[This use method](https://github.com/maratvmk/rails/blob/783deae99a4850f597135146b19e7ee4622da94e/activesupport/lib/active_support/core_ext/module/setup.rb#L9) was a bit intrusive.
Therefore, it quite made sense to test that reincluding a module when it has already been didn't just remove it.

Since we now rely on the native include method now, though, there is no use for this anymore.

cc @senny