test: remove Object.observe from tests

Testing this wasn't really useful, besides Object.observe is going to be
deprecated.

Also this test fails with Chakra (#4765) for obvious reason.

PR-URL: https://github.com/nodejs/node/pull/4769
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

test/parallel/test-microtask-queue-integration-domain.js

@@ -12,13 +12,6 @@ require('domain');
 var implementations = [
   function(fn) {
     Promise.resolve().then(fn);
-  },
-  function(fn) {
-    var obj = {};
-
-    Object.observe(obj, fn);
-
-    obj.a = 1;
   }
 ];
 

test/parallel/test-microtask-queue-integration.js

@@ -5,13 +5,6 @@ var assert = require('assert');
 var implementations = [
   function(fn) {
     Promise.resolve().then(fn);
-  },
-  function(fn) {
-    var obj = {};
-
-    Object.observe(obj, fn);
-
-    obj.a = 1;
   }
 ];