Remove traces of old GTK configs

Summary:
Since slider behavior is controlled by kded daemon, gtk kcm has no control over it now.

It appears, that some traces of the warps-slider config were not removed. Now it is removed along with some other ones redundant lines.

Test Plan: Check that GTK KCM no longer crash, if entered from System Settings.

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25863