Delete old birthday calendar object when moving contact to another address book

When an contact is moved to another address book, the contact is copied to
the second address book.
During copying, the birthday event is created - but it gets the same UID
as the contact's birthday event in the first address book.

To prevent the "Calendar object with uid already exists" error that followed,
we need to delete the old entry before the new one is created.

Resolves: https://github.com/nextcloud/server/issues/20492


Signed-off-by: Christian Weiske <cweiske@cweiske.de>