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 a7fd592d. (r4178)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@4184 e93f8b46-1217-0410-a6f0-8f06a7374b81