Fix DTD: startup_list is unused

[abouteiller]

Original report by Thomas Herault (Bitbucket: herault, GitHub: therault).

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A DTD taskpool cannot discover tasks before it is enqueued into a context, because task insertion need to know the rank of the caller.

As a consequence, parsec_dtd_schedule_tasks and the field startup_list are unused / have no effects. The code should be cleaned up.

[bosilca]

This will be addressed by bitbucket:#552.

[abouteiller]

Now PR #323