Remove obsolete ElementManager

I don't see anything in 📌 Automated Drupal 11 compatibility fixes for layout_builder_st Needs review which addresses this issue. The file in question, src/ElementManager.php, does not get changed in the merge request in that issue.

ah ok,
@liam morland, if you have a chance, please see if you can understand the test results in

📌 Automated Drupal 11 compatibility fixes for layout_builder_st Needs review

I've marked the MR as RTBC only because we need to move forward , I re-enabled automated testing in doing so and resolved a bunch of issues. With that said, more eyes and brains needed on this project. If you have any improvements, make a new MR/branch from the MR in 3431600