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

test/mjsunit/override-read-only-property.js

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-// Flags: --es5_readonly
-
 // According to ECMA-262, sections 8.6.2.2 and 8.6.2.3 you're not
 // allowed to override read-only properties, not even if the read-only
 // property is in the prototype chain.
 //
 // However, for compatibility with WebKit/JSC, we allow the overriding
 // of read-only properties in prototype chains.
 
 function F() {};
 F.prototype = Number;
 
@@ skipping 17 matching lines @@
 // Assignment to read-only property on the object itself is ignored.
 Number.MAX_VALUE = 42;
 assertEquals(original_number_max, Number.MAX_VALUE);
 
 // G should be read-only on the global object and the assignment is
 // ignored.
 (function G() {
   eval("G = 42;");
   assertTrue(typeof G === 'function');
 })();