Remove useless test header setup for emails

Once upon a time we needed a helper to set the `from` header of emails.
This is now set in the ApplicationMailer and not necessary any more.

spec/system/admin/users_spec.rb

@@ -8,7 +8,6 @@ describe "Managing users" do
 
   context "as super-admin" do
     before do
-      setup_email
       login_as_admin
     end