Remove non-existent --serialize-toplevel flag from tests

Bug: v8:7109
Change-Id: I20b7eba388415c0dd3bfe64130e6c9a7eaa3a5c8
Reviewed-on: https://chromium-review.googlesource.com/803436Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49930}

test/mjsunit/deserialize-reference.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --cache=code --serialize-toplevel
+// Flags: --cache=code
 
 var a = "123";
 assertEquals(a, "123");

test/mjsunit/serialize-embedded-error.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// --serialize-toplevel --cache=code
+// --cache=code
 
 var caught = false;
 try {

test/mjsunit/serialize-ic.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --cache=code --serialize-toplevel
+// Flags: --cache=code
 
 var foo = [];
 foo[0] = "bar";