Remove Listener namespace

We may reverse this in the future, but for now just going for
consistency between domains (the majority do not use a Listener
namespace)

src/Admin/AdminServiceProvider.php

@@ -11,7 +11,6 @@
 
 namespace Flarum\Admin;
 
-use Flarum\Admin\Listener\CheckCustomLessFormat;
 use Flarum\Admin\Middleware\RequireAdministrateAbility;
 use Flarum\Event\ConfigureMiddleware;
 use Flarum\Extension\Event\Disabled;

src/Admin/CheckCustomLessFormat.php

@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-namespace Flarum\Admin\Listener;
+namespace Flarum\Admin;
 
 use Flarum\Foundation\ValidationException;
 use Flarum\Settings\Event\Serializing;

src/User/SelfDemotionGuard.php

@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-namespace Flarum\User\Listener;
+namespace Flarum\User;
 
 use Flarum\Group\Group;
 use Flarum\User\Event\Saving;

src/User/UserServiceProvider.php

@@ -78,7 +78,7 @@ class UserServiceProvider extends AbstractServiceProvider
 
         $events = $this->app->make('events');
 
-        $events->subscribe(Listener\SelfDemotionGuard::class);
+        $events->subscribe(SelfDemotionGuard::class);
         $events->subscribe(EmailConfirmationMailer::class);
         $events->subscribe(UserMetadataUpdater::class);
         $events->subscribe(UserPolicy::class);