Stub current_user in all component tests

The `sign_in(nil)` method was a bit hard to understand IMHO. After all,
in controller and system tests we don't have to specify no user is
signed in; that's the default behavior.

So we're making it the default behavior for component tests as well.

spec/components/machine_learning/comments_summary_component_spec.rb

@@ -7,7 +7,6 @@ describe MachineLearning::CommentsSummaryComponent do
   before do
     Setting["feature.machine_learning"] = true
     Setting["machine_learning.comments_summary"] = true
-    sign_in(nil)
   end
 
   it "is displayed when the setting is enabled" do