Remove --experimental-classes flag and related dead code.

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index 75b73829e3270c491a00de5d69732643ddb73d99..a60b218a37ba1dab589294890e288f8ff1ed6e86 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -319,8 +319,7 @@ class ParserBase : public Traits {
            kind == kNormalFunction);
     Scope* result =
         new (zone()) Scope(zone(), parent, scope_type, ast_value_factory());
-    bool uninitialized_this =
-        FLAG_experimental_classes && IsSubclassConstructor(kind);
+    bool uninitialized_this = IsSubclassConstructor(kind);
     result->Initialize(uninitialized_this);
     return result;
   }