SimpleKCM: Remove header separator

This MR removes the header separator in SimpleKCM, fixing a duplicate separator shown at the top of the page when the page content does not fit the scroll view. This is seen in the Notifications KCM, and the User Feedback KCM when the slider is maxed out.

In Plasma Welcome, this is also seen in the User Feedback page only when the scrolling content does not fit and as such appears/disappears when changing the size of the window. For Plasma Welcome, we can do the separator ourselves (already one is added for content-based KCMs such as accounts). This is probably appropriate as the KCMs represent embedded content and should probably get a separator all the time, not just when the content is long enough to scroll.