It seems as if these buttons are not adjustable via the config.
The buttons
- Finder: added if the users group has Finder-privileges
- Assets: always added
are initially specified in /modules/Cockpit/assets/components/field-html.tag.
The button
- CollectionLink: always added
is added in /modules/Collections/assets/link-collectionitem.js
App.$(document).on('init-html-editor', function (e, editor) {
One solution would be, to create your own custom field type ( use the field-html.tag file as base, edit it to your liking and copy it into config/tags/).