- Introduces a `unshare` method in `CalDavBackend` to handle user unshares.
- Implements check to determine if unshare entry is needed based on group/circle membership.
- Ensures `updateShares` is only used when the calendar owner manages shares.
- Resolves issue where unsharing a calendar as owner created an unshare entry in `oc_dav_shares`.
Related PRs:
- https://github.com/nextcloud/server/pull/43117
- https://github.com/nextcloud/server/pull/47737
Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>
