Remove unneeded test for data-reactroot

This is an internal attribute of React and it is removed since React 16.
Despite this, it gets added in some cases, specially when we use the
bundled react with Sprocket. We don't need to depend our tests on the
existance of such attributes

Author: ahangarha

Diff for: test/react/server_rendering/bundle_renderer_test.rb

@@ -36,7 +36,6 @@ class BundleRendererTest < ActiveSupport::TestCase
       result = @renderer.render("Todo", { todo: "write tests" }, nil)
 
       assert_match(%r{<li.*write tests</li>}, result)
-      assert_match(/data-reactroot/, result)
     end
 
     test "#render accepts strings" do