Don't allow "Start Time" / "End Time" in Calendar event forms to be locked or have defaults assigned

Summary: Fixes T11733. This fixes the issue by working around it, but it isn't useful to set these fields to a default value anyway.

Test Plan: Created a default Calendar form, set some other defaults, created an event, stuff no longer exploded.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11733

Differential Revision: https://secure.phabricator.com/D16753