Replace IncidenceBase::deleteAttendee with setAttendee()

Summary:
deleteAttendee is fortunately unused, and relies on the fact that Attendee
is a pointer here.

setAttendee will enable porting code that currently modifies existing
attendees of an incidence via pointer access.

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21537