[Colors KCM] Only move to next scheme if the selected one is marked for deletion

Summary:
No need to select another scheme if the scheme that is to be removed is not
selected.

Test Plan: Mark a scheme other than the selected one for deletion

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26522