[parser] Remove forward declarations for dead class DuplicateFinder

Change-Id: Ib584fa4fa38d1f042dc7220f516e35c0619ace45
Reviewed-on: https://chromium-review.googlesource.com/c/1350127
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57827}

src/parsing/expression-classifier.h

@@ -13,7 +13,6 @@
 namespace v8 {
 namespace internal {
 
-class DuplicateFinder;
 template <typename T>
 class ZoneList;
 

src/parsing/scanner.h

@@ -24,7 +24,6 @@ namespace internal {
 
 class AstRawString;
 class AstValueFactory;
-class DuplicateFinder;
 class ExternalOneByteString;
 class ExternalTwoByteString;
 class ParserRecorder;