Remove "at unknown-time" from poll results.

This was broken by an incidental change in #826 (reimplementation of sequential task type).  The "at TIME" suffix is supposed to be removed from poll results to turn them into normal task messages for subsequent processing.  If a task gets killed so that there is no EXIT line in the status file the poll contains *unknown-time* rather than the specific time string format being matched - so the poll result is logged but does not cause the task proxy to change state to *failed*.

@matthewrmshin - please review.