Better fix for "unarchived WAL files get deleted on crash recovery" bug.

Revert my earlier fix for the bug that unarchived WAL files get deleted on
crash recovery, commit c9cc7e05. We create
a .done file for files streamed or restored from archive, so the WAL file
recycling logic used during normal operation works just as well during
archive recovery.

Per Fujii Masao's suggestion.