Remove @todo linking to closed issue in onfigureBlockFormBase::doBuildForm

Problem/Motivation

In #3123832: [META] Fix @todo items referencing closed issues we discovered that there's a @todo in \Drupal\layout_builder\Form\ConfigureBlockFormBase::doBuildForm which references a d.o. issue that is already closed; #2268787: Block plugin forms should not rely on being called from \Drupal\block\BlockForm

Here's the @todo:

    // @todo Remove once https://www.drupal.org/node/2268787 is resolved.
    $form_state->set('block_theme', $this->config('system.theme')->get('default'));

Steps to reproduce

Proposed resolution

At looks like, at least to me, we dropped 'block_theme' from the form state in #2268787: Block plugin forms should not rely on being called from \Drupal\block\BlockForm.
See also the related CR: https://www.drupal.org/node/3221663

I think we can safely remove the line of code below the @todo

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet