Remove dead code from JsonApiDocumentTopLevelNormalizerTest

Problem/Motivation

Follow up #2922121: When fetching related resources, getResourceType() in CurrentContext gets the wrong resource type. to remove

$this->markTestIncomplete('This fails and should be fixed by https://www.drupal.org/project/drupal/issues/2922121');

from \Drupal\Tests\jsonapi\Kernel\Normalizer\JsonApiDocumentTopLevelNormalizerTest::testNormalizeRelated.

Steps to reproduce

Proposed resolution

Remaining tasks

Patch
Fix test
Review
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User interface changes

API changes

Data model changes

Release notes snippet

Problem/Motivation

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet