Remove --harmony-tostring runtime flag

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index c7b7cc5abc8e26985dc7d6cc9418c4b80199f8d0..fa72f425c442cc313babf527170c3dae31107581 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -2251,7 +2251,6 @@ void Bootstrapper::ExportExperimentalFromRuntime(Isolate* isolate,
                           isolate->factory()->ToBoolean(FLAG), NONE); \
   }
 
-  INITIALIZE_FLAG(FLAG_harmony_tostring)
   INITIALIZE_FLAG(FLAG_harmony_species)
 
 #undef INITIALIZE_FLAG
@@ -2293,13 +2292,6 @@ void InstallPublicSymbol(Factory* factory, Handle<Context> native_context,
 }
 
 
-void Genesis::InitializeGlobal_harmony_tostring() {
-  if (!FLAG_harmony_tostring) return;
-  InstallPublicSymbol(factory(), native_context(), "toStringTag",
-                      factory()->to_string_tag_symbol());
-}
-
-
 void Genesis::InitializeGlobal_harmony_regexp_subclass() {
   if (!FLAG_harmony_regexp_subclass) return;
   InstallPublicSymbol(factory(), native_context(), "match",
@@ -2957,7 +2949,6 @@ bool Genesis::InstallExperimentalNatives() {
   static const char* harmony_modules_natives[] = {nullptr};
   static const char* harmony_regexps_natives[] = {"native harmony-regexp.js",
                                                   nullptr};
-  static const char* harmony_tostring_natives[] = {nullptr};
   static const char* harmony_iterator_close_natives[] = {nullptr};
   static const char* harmony_sloppy_natives[] = {nullptr};
   static const char* harmony_sloppy_function_natives[] = {nullptr};