Remove unused legacy/hidden RecentTaskInfo fields

- Nothing references a recent task's childTaskInfos or
  lastSnapshotData so we can remove this code and skip the work of
  storing the last snapshot metadata or computing task infos for all
  children

Bug: 346588978
Flag: EXEMPT internal cleanup
Test: atest RecentTasksTest
Change-Id: Ie45b4056ae6d0fed20d27e24ae655927e6d79cf2