[stubs] Further optimize Load/StoreGlobalViaContext stubs.

src/contexts.cc

Index: src/contexts.cc
diff --git a/src/contexts.cc b/src/contexts.cc
index 88f593fab8525a197cc2d13d47dbdf8ece4a3292..31ce98c8986b402bba38a1d13a1f87be9b2f6d9e 100644
--- a/src/contexts.cc
+++ b/src/contexts.cc
@@ -365,8 +365,6 @@ void Context::InitializeGlobalSlots() {
     int context_locals = scope_info->ContextLocalCount();
     int index = Context::MIN_CONTEXT_SLOTS + context_locals;
     for (int i = 0; i < context_globals; i++) {
-      // Clear both read and write slots.
-      set(index++, empty_cell);
       set(index++, empty_cell);
     }
   }