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.