Revert "git stash: avoid data loss when "git stash save" kills a directory"

This reverts commit a73653130edd6a8977106d45a8092c09040f9132, as it
has been reported that "ls-files --killed" is too time-consuming in
a deep directory with too many untracked crufts (e.g. $HOME/.git
tracking only a few files).

We'd need to revisit it later but "ls-files --killed" needs to be
optimized before it happens.