FIX: Multiple channels marked active in sidebar (#21045)

Followup to c1dc6a2db4,
this commit just missed removing one of the @computed
decorators which was causing multiple active channels
to show in the sidebar. Fix the issue and introduce a
system spec to catch this.

plugins/chat/assets/javascripts/discourse/initializers/chat-sidebar.js

@@ -9,7 +9,6 @@ import { emojiUnescape } from "discourse/lib/text";
 import { decorateUsername } from "discourse/helpers/decorate-username-selector";
 import { until } from "discourse/lib/formatter";
 import { inject as service } from "@ember/service";
-import { computed } from "@ember/object";
 
 export default {
   name: "chat-sidebar",
@@ -196,7 +195,6 @@ export default {
               return this.channel.slugifiedTitle;
             }
 
-            @computed("chatService.activeChannel")
             get classNames() {
               const classes = [];