regenerate unique id of date when importing dates, fixes #3103

Closes #3103

Merge request studip/studip!3272

lib/classes/calendar/ICalendarImport.php

@@ -368,7 +368,6 @@ class ICalendarImport
             $date->chdate = $date->mkdate;
         }
         $date->import_date = $this->import_time;
-        $date->unique_id = $properties['UID'];
 
         $this->setRecurrenceRule($date, $properties['RRULE']);
         $date->store();