Drop deprecated --es5-readonly flag from test suite.

test/mjsunit/readonly.js

 // Copyright 2011 the V8 project authors. All rights reserved.
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-// Flags: --allow-natives-syntax --es5_readonly
-// Flags: --harmony-proxies
+// Flags: --allow-natives-syntax --harmony-proxies
 
 // Different ways to create an object.
 
 function CreateFromLiteral() {
   return {};
 }
 
 function CreateFromObject() {
   return new Object;
 }
@@ skipping 184 matching lines @@
     var o = c();
     Assign2(o, i);
     assertEquals(i, o.a);
   }
   %OptimizeFunctionOnNextCall(Assign2);
   ReadonlyByNonwritableDataProperty(p, "a");
   var o = c();
   Assign2(o, 0);
   assertTrue(o.a < 0);
 })();