Revert "prune: --grace=time"

Revert "prune: --grace=time"

This reverts commit 9b088c4e394df84232cfd37aea78349a495b09c1.

Protecting 'mature' objects does not make it any safer.  We should
admit that git-prune is inherently unsafe when run in parallel with
other operations without involving unwarranted locking overhead,
and with the latest git, even rebase and reset would not immediately
create crufts anyway.