SimpleKCM: remove custom header and footer handling
This code is completely unnecessary because the component inherits from Kirigami.ScrollablePage, which already knows how to handle headers and footers. Currently this code has the effect of inappropriately applying page margins to header and footer elements. Removing the custom code fixes that bug and results in no visible regressions.
Now KCM.SimpleKCM's only purpose is to automatically set the title and draw lines above or below the view when it's scrollable, on the assumption that the view as well as its headers and footers are all using the Window background color. We should consider removing it in KF6 and upstreaming the scroll separator functionality into Kirigami.ScrollablePage, if needed.