DropPendingCollectionReaper was introduced during PM-716 (legacy 2 phase drop which requires renaming the dropped collection to system.drops.<collection_name>).

Later that style was replaced by KVDropPendingIdentReaper and PM-1114 (making WT to support 2  phase drop). We will not support storage engines other than WT in the near future, so we should remove DropPendingCollectionReaper and its usage.

Doing this may also get rid of some performance overheads as currently this code is called every time committed snapshot is updated, and it involves taking a mutex (though this is very minor since the mutex isn't contented).