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

spec/models/draft_spec.rb

@@ -12,6 +12,22 @@ describe Draft do
     Fabricate(:post)
   end
 
+  context 'system user' do
+    it "can not set drafts" do
+      # fake a sequence
+      DraftSequence.create!(user_id: Discourse.system_user.id, draft_key: "abc", sequence: 10)
+
+      seq = Draft.set(Discourse.system_user, "abc", 0, { reply: 'hi' }.to_json)
+      expect(seq).to eq(0)
+
+      draft = Draft.get(Discourse.system_user, "abc", 0)
+      expect(draft).to eq(nil)
+
+      draft = Draft.get(Discourse.system_user, "abc", 1)
+      expect(draft).to eq(nil)
+    end
+  end
+
   context 'backup_drafts_to_pm_length' do
     it "correctly backs up drafts to a personal message" do
       SiteSetting.backup_drafts_to_pm_length = 1