realm: sync: allow explicit 'none' for 'remove-vanished' option

realm: sync: allow explicit 'none' for 'remove-vanished' option

with that, the api call can now override the default option
that is set on the realm (if any) by providing 'none'

it was not possible previously to override the realm default
when one wanted no properties to delete

no other code changes are necessary since we only extract the
known values 'acl' etc. and 'none' has no meaning there

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>