Remove obsolete ElementManager

Problem/Motivation

The comments for Drupal\layout_builder_st\ElementManager include:

@todo Remove if https://www.drupal.org/node/2987208 → is fixed.

#2987208: ElementInfoManager does not provide an alter hook, cannot alter element plugin definitions → has been fixed for several years.

Proposed resolution

Determine if the comment is valid. If so, make the appropriate removals. Otherwise, update or remove the comment.

Remaining tasks

Implement.

User interface changes

None.

API changes

None.

Data model changes

None.