Make rubocop happy and use defined? when checking for reviewable_api_enabled

plugin.rb

@@ -35,19 +35,12 @@ after_initialize do
     - flagged-* connectors
     - flagged queue code inside the JS initializer
 =end
-  reviewable_api_enabled = begin
-    'Reviewable'.constantize
-    true
-  rescue NameError
-    false
-  end
+  reviewable_api_enabled = defined?(Reviewable)
 
   # Raise an invalid access error if a user tries to act on something
   # not assigned to them
   DiscourseEvent.on(:before_staff_flag_action) do |args|
-    return if reviewable_api_enabled
-
-    if SiteSetting.assign_locks_flags?
+    if !reviewable_api_enabled && SiteSetting.assign_locks_flags?
 
       if custom_fields = args[:post].topic&.custom_fields
         if assigned_to_id = custom_fields[TopicAssigner::ASSIGNED_TO_ID]