FIX: properly ban non human users from draft system

Previously we had a partial fix in place where non human users
were not allowed draft sequences, this left edges around where non
human users asked for drafts yet had none.

For example system could already have a few drafts in place.

This also removes and extensibility point we added that is not in use

app/models/draft_sequence.rb

@@ -2,10 +2,12 @@
 
 class DraftSequence < ActiveRecord::Base
   def self.next!(user, key)
+    return nil if !user
+
     user_id = user
     user_id = user.id unless user.is_a?(Integer)
 
-    return 0 if invalid_user_id?(user_id)
+    return 0 if !User.human_user_id?(user_id)
 
     h = { user_id: user_id, draft_key: key }
     c = DraftSequence.find_by(h)
@@ -18,27 +20,17 @@ class DraftSequence < ActiveRecord::Base
   end
 
   def self.current(user, key)
-    return nil unless user
+    return nil if !user
 
     user_id = user
     user_id = user.id unless user.is_a?(Integer)
 
-    return 0 if invalid_user_id?(user_id)
+    return 0 if !User.human_user_id?(user_id)
 
     # perf critical path
     r, _ = DB.query_single('select sequence from draft_sequences where user_id = ? and draft_key = ?', user_id, key)
     r.to_i
   end
-
-  cattr_accessor :plugin_ignore_draft_sequence_callbacks
-  self.plugin_ignore_draft_sequence_callbacks = {}
-
-  def self.invalid_user_id?(user_id)
-    user_id < 0 || self.plugin_ignore_draft_sequence_callbacks.any? do |plugin, callback|
-      plugin.enabled? ? callback.call(user_id) : false
-    end
-  end
-  private_class_method :invalid_user_id?
 end
 
 # == Schema Information