Simplify AggregatedFetchScope code, removing all setters.

Summary:
The naming was confusing (those were not setters but a counter
increase/decrease -- if you call a setter with the same value twice, you
expect the second call to be a no-op), and they were used from a single
place anyway.

Also remove unused methods (add/remove).

Test Plan: notificationmanagertest and aggregatedfetchscopetest still pass

Reviewers: dvratil

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D18711