FIX: Memory Leaks when decorating posts (#7749)

* Remove long-deprecated method

* FIX: Memory Leaks when decorating posts

Previously we'd keep creating mixins dynamically when decorating the
same class.

This code changes the API to recommend an `id` parameter for each
decorator which will avoid leaks. All plugins should be updated to
include this parameter, although if they don't in the meantime it'll
just mean a warning in the console (and a continued leak.)

plugins/discourse-details/assets/javascripts/initializers/apply-details.js.es6

@@ -1,7 +1,9 @@
 import { withPluginApi } from "discourse/lib/plugin-api";
 
 function initializeDetails(api) {
-  api.decorateCooked($elem => $("details", $elem).details());
+  api.decorateCooked($elem => $("details", $elem).details(), {
+    id: "discourse-details"
+  });
 
   api.addToolbarPopupMenuOptionsCallback(() => {
     return {