FIX: Allow users to select "regular" categories (#16857)

Categories that had a CategoryUser record and the notification level
set to "Normal" were not selectable in any of the "Watched", "Tracked",
"Watching First Post" or "Muted" inputs. This happened because the
category seemed to be already selected in the "Normal" input, but that
does not exist (it is the default value if category is not present in
any of the other inputs).

app/assets/javascripts/discourse/app/controllers/preferences/categories.js

@@ -20,13 +20,10 @@ export default Controller.extend({
     "model.watchedCategories",
     "model.watchedFirstPostCategories",
     "model.trackedCategories",
-    "model.regularCategories",
     "model.mutedCategories"
   )
-  selectedCategories(watched, watchedFirst, tracked, regular, muted) {
-    return []
-      .concat(watched, watchedFirst, tracked, regular, muted)
-      .filter((t) => t);
+  selectedCategories(watched, watchedFirst, tracked, muted) {
+    return [].concat(watched, watchedFirst, tracked, muted).filter((t) => t);
   },
 
   @discourseComputed

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

@@ -178,7 +178,7 @@ export default {
       skip_new_user_tips: false,
       enable_quoting: true,
       muted_category_ids: [],
-      regular_category_ids: [],
+      regular_category_ids: [4],
       tracked_category_ids: [],
       watched_category_ids: [3],
       watched_first_post_category_ids: [],