Remove auto-generated TinyMCE files

These files are already being copied from `node_modules/` to `public/` whenever we run `rails s` (see `config/initializers/assets.rb`).

Author: aaronskiba

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 - Fix failing eslint workflow / upgrade `actions/checkout` & `actions/setup-node` to v3 [#3503](https://github.com/DMPRoadmap/roadmap/pull/3503)
 - Fix rendering of `confirm_merge` partial [#3515](https://github.com/DMPRoadmap/roadmap/pull/3515)
+- Remove Auto-Generated TinyMCE Skins and Add `public/tinymce/skins/` to `.gitignore` [#3466](https://github.com/DMPRoadmap/roadmap/pull/3466)
 
 ## v5.0.0