Don't get hit forever by an old setting from baloorc when hand-editing the agent rc file.

Summary: i.e. the migration should only happen once.

Test Plan: removing akonadi_indexing_agentrc now triggers a full reindexing as expected.

Reviewers: dvratil, vkrause

Reviewed By: vkrause

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D5349