do not create /cluster/<cid> unconditionally

do not create /cluster/<cid> unconditionally

while looking through the spooldir creation we noticed the mkdir call
on a relative path. This creates a '/cluster/<cid>/' directory on each system
which has a cluster.conf (<cid> being the node's clusterid). This is not used
since the spooldirs are in '/var/spool/pmg/cluster/'

Simply drop the mkdir call, since the spooldirs get created upon cluster
creation (PMG::API2::Cluster::create) and joining to an existing cluster.

Reported-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>