Remove nyan_cat_init() and the legacy group and expected deprecation from \Drupal\Tests\system\Functional\Theme\EngineNyanCatTest::testTemplateOverride

Problem/Motivation

hook_init() is deprecated for theme engines and will be removed in Drupal 10. This issue will do the removal and clean up the tests and the test theme engine.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet