The following lines can be removed from core/assets/vendor/ckeditor/build-config.js.
'dialog' : 1,
'dialogui' : 1,
'menu' : 1,
'floatpanel' : 1,
'panel' : 1,
'button' : 1,
'popup' : 1,
'richcombo' : 1,
'listblock' : 1,
'fakeobjects' : 1,This should be double-checked with the CKEditor developers shortly before release.