DEV: Nuke plugin related code from core (#20928)

Plugin related code should be not polluting core's code base.

app/assets/javascripts/discourse/app/components/reviewable-item.hbs

@@ -68,12 +68,7 @@
     {{/if}}
   </div>
 
-  {{#if
-    (or
-      (eq this.reviewable.type "ReviewableFlaggedPost")
-      (eq this.reviewable.type "ReviewableChatMessage")
-    )
-  }}
+  {{#if (eq this.reviewable.type "ReviewableFlaggedPost")}}
     {{#if (eq this.reviewable.status 0)}}
       <h3 class="reviewable-item__context-question">
         {{this.reviewable.flaggedPostContextQuestion}}

app/assets/javascripts/discourse/tests/fixtures/session-fixtures.js

@@ -25,7 +25,6 @@ export default {
       custom_fields: {},
       muted_category_ids: [],
       dismissed_banner_key: null,
-      akismet_review_count: 0,
       can_review: true,
       ignored_users: [],
       groups: [

app/assets/stylesheets/common/base/reviewables.scss

@@ -232,12 +232,7 @@
       color: var(--secondary);
       border-radius: 8px;
       background-color: var(--secondary-high);
-      &.-flagged-post,
-      &.-flagged-chat-message,
-      &.-akismet-flagged-post,
-      &.-akismet-flagged-user,
-      &.-ai-flagged-post,
-      &.-ai-flagged-chat-message {
+      &.-flagged-post {
         background-color: var(--danger-medium);
       }
       &.-queued-post,