Replace the excludeRange mode setting when already available

Summary:
Instead of appending a new entry.

BUG: 244857
BUG: 408415

Test Plan:
- remove fonts.conf
- start fonts kcm
- enable exclude range
- save
- see config is added only once
- modify config
- save
- see config is replaced and not added again
- disable exclude range
- save
- see config is totally removed.

Reviewers: fvogt

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21748