Follow-up for #2821077: address forgotten @todo in UserTest::testPatchDxForSecuritySensitiveBaseFields()

Problem/Motivation

#2821077: PATCHing entities validates the entire entity, also unmodified fields, so unmodified fields can throw validation errors was solved.

But UserTest::testPatchDxForSecuritySensitiveBaseFields() still contains:

// @todo Remove the array_diff_key() call in https://www.drupal.org/node/2821077

Proposed resolution

Address the todo.

Remaining tasks

Review.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.