Revert setting default date for news

Based on a discussion in this ticket[1] we've decided to revert this for
now, pending a full discussion about how to treat this validation.

Revert "Default date is current time for non-imported news". This
reverts commit 08e8f8cfc355b25973b41f81b38da0824913da17.

[1] https://www.pivotaltracker.com/story/show/44021041