Republishing loses student MC Question answers in portal. [#66299458]

On line 313 of activity_runtime_api.rb we were deleting the Choices from
the database, however the active record entities still 'existed' with all
their previous values (such as ID), but the attributes were frozen, and
the DB table was empty. A longer discussion can be found here:
(see delete) http://api.rubyonrails.org/v4.0.3/classes/ActiveRecord/Persistence.html
and here: http://bit.ly/1oN8AMb

"Republishing causes previously answered MC questions in portal
report to "lose" student answers."

https://www.pivotaltracker.com/story/show/66299458