It was introduced by 98e4d39 ("progress: add speed format" 2011-5-9) and was
intended to hide ETA information for the first few seconds.
Later 5d261fd ("progress: add a changedelay to prevent parallel topics from
flapping (issue2698)" 2011-6-23) introduced changedelay config which hides
the entire progress bar for the first few seconds. So progress.estimate seems
somehow duplicated feature-wise. Since it's experimental and duplicated, let's
just remove it. This makes the next patch simpler - it no longer needs to make
sure starttimes is the real start time.