Remove outdated comment

Library/Homebrew/cmd/doctor.rb

@@ -1021,10 +1021,6 @@ def check_for_unlinked_but_not_keg_only
     end
   end.map{ |pn| pn.basename }
 
-  # NOTE very old kegs will be linked without the LinkedKegs symlink
-  # this will trigger this warning but it's wrong, we could detect that though
-  # but I don't feel like writing the code.
-
   if not unlinked.empty? then <<-EOS.undent
     You have unlinked kegs in your Cellar
     Leaving kegs unlinked can lead to build-trouble and cause brews that depend on