remove unused plugins from docs

* twbs#25319
* twbs#25320
* twbs#25784

site/docs/4.2/migration.md

@@ -309,10 +309,7 @@ Removed the following mixins: `animation`, `animation-delay`, `animation-directi
 
 Our documentation received an upgrade across the board as well. Here's the low down:
 
-- We're still using Jekyll, but we have plugins in the mix:
-  - `bugify.rb` is used to efficiently list out the entries on our [browser bugs]({{ site.baseurl }}/docs/{{ site.docs_version }}/browser-bugs/) page.
-  - `example.rb` is a custom fork of the default `highlight.rb` plugin, allowing for easier example-code handling.
-  - `callout.rb` is a similar custom fork of that, but designed for our special docs callouts.
+- We're still using Jekyll
 - All docs content has been rewritten in Markdown (instead of HTML) for easier editing.
 - Pages have been reorganized for simpler content and a more approachable hierarchy.
 - We moved from regular CSS to SCSS to take full advantage of Bootstrap's variables, mixins, and more.