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

test/mjsunit/object-define-property.js

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 // Tests the object.defineProperty method - ES 15.2.3.6
 
-// Flags: --allow-natives-syntax --es5-readonly
+// Flags: --allow-natives-syntax
 
 // Check that an exception is thrown when null is passed as object.
 var exception = false;
 try {
   Object.defineProperty(null, null, null);
 } catch (e) {
   exception = true;
   assertTrue(/called on non-object/.test(e));
 }
 assertTrue(exception);
@@ skipping 1156 matching lines @@
 Assign(new C);
 
 // Test that changes to the prototype of a simple constructor are not ignored,
 // even after creating initial instances.
 function C() {
   this.x = 23;
 }
 assertEquals(23, new C().x);
 C.prototype.__defineSetter__('x', function(value) { this.y = 23; });
 assertEquals(void 0, new C().x);