Revert "Fixed: gantt broken when no due date on project issues and versions."
It broke the tests for overdue projects and isn't a good way to fix #6350
because it makes an assumption that a project is due Today and will still
error if issue tracking is disabled.
This reverts commit a7fd592db4037d635d04ca5ce3f01d0af3f0c8ae. (r4178)
