Revert "Add POST support to XHR as well as .status and statusText support"

sky/tests/framework/xmlhttprequest/xhr-does-not-exist.sky

-<sky>
-<import src="/sky/tests/resources/chai.sky" />
-<import src="/sky/tests/resources/mocha.sky" />
-<import src="/sky/framework/xmlhttprequest.sky" as="XMLHttpRequest" />
-<script>
-describe("xmlhttprequest", function() {
-  it("should call onerror when endpoint does not exist", function(done) {
-    var xhr = new XMLHttpRequest();
-    xhr.open("GET", "does_not_exist.html");
-    xhr.onerror = function() {
-      assert.fail("onload", "onerror", "onerror should not be called.");
-      done();
-    }
-    xhr.onload = function() {
-      // Missing files are application-level errors, not network errors
-      // so onload fires, not onerror.
-      assert.equal(xhr.status, 404);
-      assert.equal(xhr.statusText, "HTTP/1.1 404 Not Found",
-          "status text should also be 404");
-      done();
-    }
-    xhr.send();
-  });
-});
-</script>
-</sky>