Remove outdated comment

Library/Homebrew/os/mac.rb

@@ -145,8 +145,6 @@ module OS
     alias_method :gcc_build_version, :gcc_42_build_version
 
     def llvm_build_version
-      # for Xcode 3 on OS X 10.5 this will not exist
-      # NOTE may not be true anymore but we can't test
       @llvm_build_version ||=
         if (path = locate("llvm-gcc")) && path.realpath.basename.to_s !~ /^clang/
           %x{#{path} --version}[/LLVM build (\d{4,})/, 1].to_i