Remove PrivateUsers=yes from paccache.service

As `PrivateUsers=yes` sets up a separate namespace (also for root), all
files are therefore owned by `nobody` to the paccache script and it is
not able to remove any files.

See `PrivateUsers=` in for further
1 job for !13 with paccache_no_privateusers in 16 seconds (queued for 2 seconds)
merge request