Formula#file_modfiled?: remove unnecessary git dir check
Formula#file_modfiled?: remove unnecessary git dir check

It's already covered by `$?.exitstatus == 0` in below.

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