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