Delete deleted events

It seems that events deleted in Untis are not always deleted in Aleksis. Example: Katharineum, Mon. 20.12.21, Ea-d, 5th/6th lesson.

assigned to @hansegucker

moved from AlekSIS-App-Alsijil#196 (moved)

assigned to @hansegucker

added part::backend prio::1 source::customer type::bug labels

created merge request !102 (merged) to address this issue

mentioned in merge request !102 (merged)

closed with merge request !102 (merged)

mentioned in commit 7d215d70