Select correct index without timer

Summary:
remove the timer that selects the index at startup
that caused flickering and things moving around since
index 0 got selected, then moved to the correct one only after a while.

Test Plan:
correct theme gets highlighed at startup
probably i'll port l&f kcm currentSelectedIndex property to indexOf
invokable as well.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3533