cleanup: speed up rm_DS_Store by multithread
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 <9a05244150b861e40c78843b801eed71bca54eac@me.com>

muellermartin/dist,jmsundar/brew,MikeMcQuaid/brew,AnastasiaSulyagina/brew,EricFromCanada/brew,reitermarkus/brew,ilovezfs/brew,jmsundar/brew,tonyg/homebrew,staticfloat/brew,claui/brew,konqui/brew,amar-laksh/brew_sudo,amar-laksh/brew_sudo,Linuxbrew/brew,pseudocody/brew,DomT4/brew,MikeMcQuaid/brew,palxex/brew,MikeMcQuaid/brew,rwhogg/brew,bfontaine/brew,hanxue/linuxbrew,maxim-belkin/brew,EricFromCanada/brew,reitermarkus/brew,palxex/brew,zmwangx/brew,pseudocody/brew,AnastasiaSulyagina/brew,mistydemeo/homebrew,mahori/brew,mgrimes/brew,nandub/brew,claui/brew,nandub/brew,gregory-nisbet/brew,mistydemeo/homebrew,aw1621107/brew,Homebrew/brew,sjackman/homebrew,vitorgalvao/brew,muellermartin/dist,palxex/brew,mistydemeo/homebrew,vitorgalvao/brew,Homebrew/brew,claui/brew,mgrimes/brew,maxim-belkin/brew,gordonmcshane/homebrew,MikeMcQuaid/brew,hanxue/linuxbrew,staticfloat/brew,EricFromCanada/brew,aw1621107/brew,Homebrew/brew,vitorgalvao/brew,konqui/brew,JCount/brew,rwhogg/brew,alyssais/brew,tonyg/homebrew,aw1621107/brew,mahori/brew,konqui/brew,ilovezfs/brew,mahori/brew,AnastasiaSulyagina/brew,pseudocody/brew,hanxue/linuxbrew,mahori/brew,gordonmcshane/homebrew,jmsundar/brew,toonetown/homebrew,toonetown/homebrew,staticfloat/brew,DomT4/brew,DomT4/brew,JCount/brew,sjackman/homebrew,Linuxbrew/brew,zmwangx/brew,maxim-belkin/brew,muellermartin/dist,rwhogg/brew,DomT4/brew,bfontaine/brew,bfontaine/brew,toonetown/homebrew,JCount/brew,Linuxbrew/brew,sjackman/homebrew,vitorgalvao/brew,nandub/brew,konqui/brew,alyssais/brew,amar-laksh/brew_sudo,mgrimes/brew,gregory-nisbet/brew,Linuxbrew/brew,claui/brew,JCount/brew,reelsense/brew,alyssais/brew,sjackman/homebrew,tonyg/homebrew,nandub/brew,ilovezfs/brew,reelsense/brew,Homebrew/brew,reitermarkus/brew,reelsense/brew,gordonmcshane/homebrew,EricFromCanada/brew,zmwangx/brew,gregory-nisbet/brew,reitermarkus/brew