kcms/landingpage: remove dead help page button
The button just leads to a "Documentation not found". I haven't been able to locate the source of that kcontrol/landingpage/index.html anywhere. Ideally we would have such a help page of course, but until that actually happens, let's remove this button as it is currently useless, and clicking on "Help" on the very first page seen in system settings leading to something broken is not a good first experience.