Drop kconf_update module, handled by kde-gtk-config now

Summary: D26891 introduces a kconf_update script which achieves the same.

Test Plan:
Ran make, no kconf_update anymore.
Default theme is still Breeze.

Reviewers: #plasma, gikari, davidedmundson

Reviewed By: #plasma, gikari, davidedmundson

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26947