Post#expunge!: fix remove_pool! to remove posts from deleted pools.

Don't silently ignore attempts to remove posts from deleted pools.
Remove the restriction on removing posts from deleted pools instead (ref: #1109).

Fixes failure to remove posts from deleted pools during expungement.

test/unit/post_test.rb

@@ -86,15 +86,24 @@ class PostTest < ActiveSupport::TestCase
           SqsService.any_instance.stubs(:send_message)
           @pool = FactoryGirl.create(:pool)
           @pool.add!(@post)
-          @post.reload
+
+          @deleted_pool = FactoryGirl.create(:pool)
+          @deleted_pool.add!(@post)
+          @deleted_pool.update_columns(is_deleted: true)
+
           @post.expunge!
+          @pool.reload
+          @deleted_pool.reload
         end
 
         should "remove the post from all pools" do
-          @pool.reload
           assert_equal("", @pool.post_ids)
         end
 
+        should "remove the post from deleted pools" do
+          assert_equal("", @deleted_pool.post_ids)
+        end
+
         should "destroy the record" do
           assert_equal([], @post.errors.full_messages)
           assert_equal(0, Post.where("id = ?", @post.id).count)