pve-cluster: dont pretend to be a time-sync provider

pve-cluster: dont pretend to be a time-sync provider

time-sync.target is a special passive unit, consumers (i.e., units that
intend to say "I want to start after synchronized time has been
established") should only order themselves after it. only providers
(i.e., units that intend to say "I am responsible for synchronizing the
clock") should pull it in via a dependency.

this and 2c7182b76f59b32efaba3135c095b9c722957af1 together also fix #998

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>