Remove mod-only bulk revert system (#4178).

The mass undo system added in #4178 is a replacement for the mod-only
bulk revert system.

test/unit/bulk_revert_test.rb

@@ -1,20 +0,0 @@
-require 'test_helper'
-
-class BulkRevertTest < ActiveSupport::TestCase
-  context "#find_post_versions" do
-    subject do
-      @user = FactoryBot.create(:user)
-      BulkRevert.new
-    end
-
-    setup do
-      subject.stubs(:constraints).returns({added_tags: ["a"]})
-      subject.expects(:query_gbq).returns([1,2,3])
-    end
-
-    should "revert all changes found in a search" do
-      q = subject.find_post_versions
-      assert_match(/post_versions\.id in \(1,2,3\)/, q.to_sql)
-    end
-  end
-end