manpages: remove no longer needed 'brew.1.md'
manpages: remove no longer needed 'brew.1.md'

Changes to the man page are made in `header.1.md`, `footer.1.md`, or the
documentation comment block in the respective command file. Remove old
Markdown file to reduce confusion, now that it is no longer needed (and
not even used by `brew man`).

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