brew: remove HOMEBREW_ON_DEBIAN7
brew: remove HOMEBREW_ON_DEBIAN7

This is not used anymore, we are building on ubuntu 16.04 for our portable packages: https://github.com/Homebrew/homebrew-portable-ruby

Homebrew/brew,MikeMcQuaid/brew,nandub/brew,nandub/brew,Homebrew/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,Homebrew/brew,nandub/brew,nandub/brew,Homebrew/brew