kded: properly delete factory to fix file descriptor leak

Not unloading the plugin loader will cause file descriptor leaks in
kded, and when fd limit is reached, accent color can't be updated
anymore because updating accent color requires writing to kdeglobals.
This fixes the leak and also fixes the accent color bug.

BUG: 458430
BUG: 458817
CCBUG: 459870
FIXED-IN: 5.26


(cherry picked from commit 3c243781)