Remove the last remaining maps using Attendee pointers as keys

Summary:
This includes two maps where Attendee pointers are technically the values,
but those maps are used bidirectionally, so those need to be changed as
well. Instead we now used the Attendee uid as key.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21688