[Fonts KCM] Remove last remnants of setNearestExistingFonts()

Summary: This cleans up after commit 0325d698.

Test Plan: make && ctest

Reviewers: #plasma, bport, broulik, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29710