Minor selser tweaks

* Don't store the serialize id in data-parsoid-serialize. It is no longer
  used.

* Add a hack to not updated the last modification status if the output from
  the wts is empty. This should be fixed properly to consider meta tags or the
  like later.

Overall 30 more selser tests are green with the hack. The ID change just makes
the output slightly less verbose.

Change-Id: Idc64687ad0db703e477fdcc8207bb5bea007ea4e
Merge "Minor selser tweaks"