[parser,ast] Simplify MarkExpressionAsAssigned.

src/parsing/preparser.h

Index: src/parsing/preparser.h
diff --git a/src/parsing/preparser.h b/src/parsing/preparser.h
index e45e2f070c7023237cb1772531486774df3e6b34..1427382d8883e038fa98a12e2b6a016c8d1a51d9 100644
--- a/src/parsing/preparser.h
+++ b/src/parsing/preparser.h
@@ -1198,11 +1198,10 @@ class PreParser : public ParserBase<PreParser> {
   V8_INLINE static void CheckAssigningFunctionLiteralToProperty(
       PreParserExpression left, PreParserExpression right) {}
 
-  V8_INLINE static PreParserExpression MarkExpressionAsAssigned(
+  V8_INLINE static void MarkExpressionAsAssigned(
       PreParserExpression expression) {
     // TODO(marja): To be able to produce the same errors, the preparser needs
     // to start tracking which expressions are variables and which are assigned.
-    return expression;
   }
 
   V8_INLINE bool ShortcutNumericLiteralBinaryExpression(PreParserExpression* x,