DEV: Get rid of old bookmark spec (#333)

The user_bookmark_serializer_spec.rb covers polymorphic bookmarks,
we no longer need this and
https://github.com/discourse/discourse/pull/16729 will
not pass without deleting it.

spec/serializers/user_bookmark_serializer_spec.rb

@@ -1,39 +0,0 @@
-# frozen_string_literal: true
-
-require 'rails_helper'
-require_relative '../support/assign_allowed_group'
-
-describe UserBookmarkSerializer do
-  fab!(:user) { Fabricate(:user) }
-  fab!(:topic) { Fabricate(:topic) }
-  fab!(:post) { Fabricate(:post, topic: topic) }
-  fab!(:bookmark) { Fabricate(:bookmark, user: user, post: post) }
-  let(:guardian) { Guardian.new(user) }
-
-  include_context 'A group that is allowed to assign'
-
-  before do
-    SiteSetting.assign_enabled = true
-    add_to_assign_allowed_group(user)
-  end
-
-  it "includes assigned user in serializer" do
-    Assigner.new(topic, user).assign(user)
-    serializer = UserBookmarkSerializer.new(bookmark, scope: guardian)
-    bookmark = serializer.as_json[:user_bookmark]
-
-    expect(bookmark[:assigned_to_user][:id]).to eq(user.id)
-    expect(bookmark[:assigned_to_user][:assign_icon]).to eq("user-plus")
-    expect(bookmark[:assigned_to_group]).to be(nil)
-  end
-
-  it "includes assigned group in serializer" do
-    Assigner.new(topic, user).assign(assign_allowed_group)
-    serializer = UserBookmarkSerializer.new(bookmark, scope: guardian)
-    bookmark = serializer.as_json[:user_bookmark]
-
-    expect(bookmark[:assigned_to_group][:id]).to eq(assign_allowed_group.id)
-    expect(bookmark[:assigned_to_group][:assign_icon]).to eq("group-plus")
-    expect(bookmark[:assigned_to_user]).to be(nil)
-  end
-end