Use constants to define resources for utterances

Self-host the JS for `utterances` is not necessary, because the list of comments cannot be loaded if the network is offline.

_data/assets/cross_origin.yml

@@ -60,6 +60,3 @@ polyfill:
 
 mathjax:
   js: https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js
-
-utterances:
-  js: https://utteranc.es/client.js

_data/assets/self_host.yml

@@ -49,6 +49,3 @@ polyfill:
 
 mathjax:
   js: /assets/lib/mathjax-3.2.0/tex-chtml.js
-
-utterances:
-  js: /assets/lib/utterances/client.js

_includes/comments/utterances.html

@@ -1,5 +1,5 @@
 <!-- https://utteranc.es/ -->
-<script src="{{ site.data.assets[origin].utterances.js }}"
+<script src="https://utteranc.es/client.js"
         repo="{{ site.comments.utterances.repo }}"
         issue-term="{{ site.comments.utterances.issue_term }}"
         crossorigin="anonymous"