brew.sh: drop obsolete source in update-preinstall
brew.sh: drop obsolete source in update-preinstall

No need to source `cmd/update.sh` if `homebrew-update` is not going to
be called directly. Amends 750bb24a787dc6e4a13f2b842a3e8dcb8d5fb761.

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