Answer
In Page Designer, expand the relevant section/group element, then expand its 'Override' options, and uncheck 'Visible' — this hides the section entirely rather than merely collapsing it, so it no longer appears on the page at all (not even as a collapsed, clickable header). This is the mechanism to reach for when you want a section gone completely, as distinct from just wanting it to start in a collapsed state while remaining available for the user to expand.