network: Drop @custom_lease_file_len variable from networkGetDHCPLeases()

network: Drop @custom_lease_file_len variable from networkGetDHCPLeases()

We don't need to track the lease file size. Instead, we can
simply check if the file was empty by comparing the buffer the
file was read into with an empty string.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>