don't delete HITs
don't delete HITs

Comment by @mizzao on PR #13:
> MTurk deletes HITs after a while (they aren't returned by `GetHIT`),
> but I'm not sure we'd want to remove them from the local database.

TurkServer/turkserver-meteor,HarvardEconCS/turkserver-meteor,VirtualLab/turkserver-meteor,HarvardEconCS/turkserver-meteor,TurkServer/turkserver-meteor,VirtualLab/turkserver-meteor,HarvardEconCS/turkserver-meteor,VirtualLab/turkserver-meteor,TurkServer/turkserver-meteor