unpack: remove dead code

Library/Contributions/cmd/brew-unpack.rb

@@ -45,10 +45,6 @@ class Formula
       do_patch
     end
   end
-
-  # handle_llvm_failure() requires extend/ENV, so let's never fail
-  # with llvm since we don't particularly care in this context.
-  def fails_with_llvm?; false; end
 end
 
 module Homebrew extend self