cleanup: speed up rm_DS_Store by multithread

Before

```
$ time brew cleanup -s
brew cleanup -s  0.73s user 3.52s system 58% cpu 7.297 total
```

After

```
$ time brew cleanup -s
brew cleanup -s  0.69s user 2.57s system 139% cpu 2.341 total
```

Closes Homebrew/homebrew#42968.

Signed-off-by: Xu Cheng <xucheng@me.com>