Remove unused EditorItemManager methods

Summary:
The methods (set)fetchScope were never used. They also violate
encapsulation: setting an arbitrary fetch scopes might lead to some
features not being fetched that are required by IncidenceDialog to work
correctly, like item tags including tag names.

Test Plan:
It compiles; projects depending on IncidenceEditor compile, too.
The methods are neither signals nor slots, so they are not used
in a string-based connect. In summary: they are actually not used
anywhere

Reviewers: #kde_pim, dvratil

Reviewed By: #kde_pim, dvratil

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D7549