Remove automatic_updates_extensions/tests/fixtures/stage_composer in favor of using StageFixtureManipulator

Problem/Motivation

follow-up from #3318625: Remove active composer fixture files in UpdaterFormTest →
Remove the staged composer metadata fixture files in\Drupal\Tests\automatic_updates_extensions\Functional\UpdaterFormTest

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes