Issue #3270438 by Wim Leers, xjm, Spokje, lauriii, nod_: Remove CKEditor 4 from core

core/.eslintrc.json

core/.eslintrc.legacy.json

core/.stylelintrc.json

core/MAINTAINERS.txt

core/assets/vendor/.gitignore

core/assets/vendor/ckeditor/CHANGES.md

core/assets/vendor/ckeditor/LICENSE.md

core/assets/vendor/ckeditor/SECURITY.md

core/assets/vendor/ckeditor/build-config.js

core/assets/vendor/ckeditor/ckeditor.js

core/assets/vendor/ckeditor/lang/_translationstatus.txt

core/assets/vendor/ckeditor/lang/af.js

core/assets/vendor/ckeditor/lang/ar.js

core/assets/vendor/ckeditor/lang/az.js

core/assets/vendor/ckeditor/lang/bg.js

core/assets/vendor/ckeditor/lang/bn.js

core/assets/vendor/ckeditor/lang/bs.js

core/assets/vendor/ckeditor/lang/ca.js

core/assets/vendor/ckeditor/lang/cs.js

core/assets/vendor/ckeditor/lang/cy.js