removed incomplete comment

as written, I don't believe this comment was helpful; I think it's
better just to steer the reader toward a general understanding of
hygiene.

src/libsyntax/fold.rs

@@ -391,23 +391,6 @@ fn fold_arg_<T: Folder>(a: &Arg, fld: &mut T) -> Arg {
 
 // build a new vector of tts by appling the Folder's fold_ident to
 // all of the identifiers in the token trees.
-//
-// This is part of hygiene magic. As far as hygiene is concerned, there
-// are three types of let pattern bindings or loop labels:
-//      - those defined and used in non-macro part of the program
-//      - those used as part of macro invocation arguments
-//      - those defined and used inside macro definitions
-// Lexically, type 1 and 2 are in one group and type 3 the other. If they
-// clash, in order for let and loop label to work hygienically, one group
-// or the other needs to be renamed. The problem is that type 2 and 3 are
-// parsed together (inside the macro expand function). After being parsed and
-// AST being constructed, they can no longer be distinguished from each other.
-//
-// For that reason, type 2 let bindings and loop labels are actually renamed
-// in the form of tokens instead of AST nodes, here. There are wasted effort
-// since many token::IDENT are not necessary part of let bindings and most
-// token::LIFETIME are certainly not loop labels. But we can't tell in their
-// token form. So this is less ideal and hacky but it works.
 pub fn fold_tts<T: Folder>(tts: &[TokenTree], fld: &mut T) -> Vec<TokenTree> {
     tts.iter().map(|tt| {
         match *tt {