Drop obsolete compiler fallback in stdenv
Drop obsolete compiler fallback in stdenv

This code originated in a slightly different form in 8e88b22fd1ec65a344ce6e4facd6dad4b415b2ad:

https://github.com/Homebrew/homebrew/blob/8e88b22fd1ec65a344ce6e4facd6dad4b415b2ad/Library/Homebrew/extend/ENV.rb#L30-L32

Back then, MacOS.default_compiler could return nil, which meant
ENV.compiler could do the same. This code was carried forward as the
surrounding code changed. At this point it should be unreachable.

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