Remove unused getter method

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/D7550