Remove [[Enumerate]] and associated reflective capabilities

[bterlson]

At the January TC39 meeting and subsequent conversations we got consensus on removal of the [[Enumerate]] MOP operation and the associated Reflect.enumerate and proxy trap enumerate. A PR implementing this consensus is here: tc39/ecma262#367.

[goyakin]

Test262 tests

[abchatra]

@Yongqu
This is failing following ESNext kangax test case as well

print((function()
{
var passed = true;
var proxy = new Proxy({}, {
  enumerate: function() {
    passed = false;
  }
});
for(var key in proxy); // Should not throw, nor execute the 'enumerate' method.
return passed;
})());

Expected: true
Actual:Exception.

[abchatra]

Removing 1.2 milestone due to backward compatibility concern. Please fix this in Master.