Revert "prune: --grace=time"

This reverts commit 9b088c4e39.

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.

t/t5700-clone-reference.sh

@@ -22,7 +22,7 @@ echo second > file2 &&
 git add file2 &&
 git commit -m addition &&
 git repack -a -d &&
-git prune --grace=off'
+git prune'
 
 cd "$base_dir"
 
@@ -56,7 +56,7 @@ echo third > file3 &&
 git add file3 &&
 git commit -m update &&
 git repack -a -d &&
-git prune --grace=off'
+git prune'
 
 cd "$base_dir"