posts: don't reparent children when expunging posts.

If an expunged post has children, just set their parent to null instead
of reparenting them. Before we reparented them to the oldest child.

app/models/post.rb

@@ -920,14 +920,7 @@ class Post < ApplicationRecord
     end
 
     def update_children_on_destroy
-      return unless children.present?
+      children.update(parent: nil)
-
-      eldest = children[0]
-      siblings = children[1..-1]
-
-      eldest.update(parent_id: nil)
-      Post.where(id: siblings).find_each { |p| p.update(parent_id: eldest.id) }
-      # Post.where(id: siblings).update(parent_id: eldest.id) # XXX rails 5
     end
 
     def update_parent_on_save

test/unit/post_test.rb

@@ -254,17 +254,6 @@ class PostTest < ActiveSupport::TestCase
           end
         end
 
-        should "reparent all children to the first child" do
-          @p1.expunge!
-          @c1.reload
-          @c2.reload
-          @c3.reload
-
-          assert_nil(@c1.parent_id)
-          assert_equal(@c1.id, @c2.parent_id)
-          assert_equal(@c1.id, @c3.parent_id)
-        end
-
         should "save a post version record for each child" do
           assert_difference(["@c1.versions.count", "@c2.versions.count", "@c3.versions.count"]) do
             @p1.expunge!
@@ -273,11 +262,6 @@ class PostTest < ActiveSupport::TestCase
             @c3.reload
           end
         end
-
-        should "set the has_children flag on the new parent" do
-          @p1.expunge!
-          assert_equal(true, @c1.reload.has_children?)
-        end
       end
     end