Remove fake-site fixture for automatic_updates_extensions

This does make \Drupal\Tests\automatic_updates_extensions\Kernel\Validator\UpdateReleaseValidatorTest pass in 📌 SupportedReleaseValidator should use ComposerInspector instead of ComposerUtility Fixed

I merged this myself

1. all of the changes are in automatic_updates_extensions/tests so it will not affect any module that is planned to be in core
2. It block 📌 SupportedReleaseValidator should use ComposerInspector instead of ComposerUtility Fixed which is core-mvp and probably would block other issue in 📌 Remove our runtime dependency on composer/composer: remove ComposerUtility Fixed which is a critical core-mvp issue

@phenaproxima feel free to re-open if you see issues or create a follow-up

Absolutely no objections.

Automatically closed - issue fixed for 2 weeks with no activity.