Allows for standard Jekyll code highlighting with distill template (alshedivat#3002)

As described in alshedivat#300,
standard Markdown code highlighting doesn't work in the Distill template
because it is clobbered by the Distill "Prism" syntax highlighting. This
PR will allow d-code tags (distill-specific code tags) to function with
the Distill syntax highlighter, but will default `highlight` and
Markdown code blocks to use the Jekyll/Rouge highlighter.

This PR also removes overrides.js, which is currently broken because
jquery isn't loaded.

Author: jacobaustin123

_layouts/distill.liquid

@@ -6,7 +6,6 @@
     <!-- Distill js -->
     <script src="{{ '/assets/js/distillpub/template.v2.js' | relative_url }}"></script>
     <script src="{{ '/assets/js/distillpub/transforms.v2.js' | relative_url }}"></script>
-    <script src="{{ '/assets/js/distillpub/overrides.js' | relative_url }}"></script>
     {% if page._styles %}
       <!-- Page/Post style -->
       <style type="text/css">