Remove a stray comment that shouldn't be here.

Author: nnethercote

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -269,7 +269,6 @@ pub(super) fn transcribe<'a>(
                 // some of the unnecessary whitespace.
                 let ident = MacroRulesNormalizedIdent::new(original_ident);
                 if let Some(cur_matched) = lookup_cur_matched(ident, interp, &repeats) {
-                    // njn: explain the use of alone here
                     let tt = match cur_matched {
                         MatchedSingle(ParseNtResult::Tt(tt)) => {
                             // `tt`s are emitted into the output stream directly as "raw tokens",