Revert "Update CKEditor 4.15.1"

ckeditor/CHANGES.md

ckeditor/build-config.js

ckeditor/ckeditor.js

ckeditor/lang/af.js

ckeditor/lang/ar.js

ckeditor/lang/az.js

ckeditor/lang/bg.js

ckeditor/lang/bn.js

ckeditor/lang/bs.js

ckeditor/lang/ca.js

ckeditor/lang/cs.js

ckeditor/lang/cy.js

ckeditor/lang/da.js

ckeditor/lang/de-ch.js

ckeditor/lang/de.js

ckeditor/lang/el.js

ckeditor/lang/en-au.js

ckeditor/lang/en-ca.js

ckeditor/lang/en-gb.js

ckeditor/lang/en.js