kcms/kfontinst: remove extra redundant separator at the bottom

Reason for the change

kcm_kfontinst adds an extra separator to the bottom of its main view, causing the bottom separator to appear as a 2px separator, inconsistent with every other page.

This MR removes the extra separator.

Test plan

I built the project, launched System Settings, went to the font management page (kcm_kfontinst), then checked how there is a 2 px gap. After the change, I repeated the same thing and see how the extra separator is now gone and visually there just appears to be a 1px separator.

Screenshots or screen recordings

Before	After

Bugs fixed

Seems to not have been noticed / reported till now.