The problem appears when you go to Administration (on top) - CMS - CMS general settings. Here when you disable the Help option then it should disappear from the top menu. However, it still appears.
The thing is that if the top menu item is Help & Support, then it is actually a link to the Questions plugin. And thus it cannot be disabled as it is a separate plugin.
However, the Help option of the CMS is related to the basic core Redmine Help functionality. It appears when the Questions plugin is installed.
So, as expected, when you disable the option then the Help feature disappears on the top menu.