Remove list of (TaskInfo, TaskMetrics) from StageInfo

From an experiment, I discovered that up to 38% of the logged bytes is
made up of StageInfo (compared to 45% for TaskEnd, which is unavoidable).
This is because the StageInfo in the StageCompleted events currently
store lists of (TaskInfo, TaskMetrics) objects, which are duplicated in
TaskEnd events anyway.

This commit gets rid of this list, which significantly cuts down on log
size and thus log time.
Remove now deprecated test