testing: Globally define logging via syslog for charon-systemd

We could make the same change for charon (actually setting it for charon
in strongswan.conf.testing would work for charon-systemd too), however,
there are dozens of test cases that currently set charondebug in
ipsec.conf.

testing/tests/tnc/tnccs-20-client-retry/hosts/dave/etc/strongswan.conf

@@ -6,9 +6,6 @@ charon-systemd {
   multiple_authentication = no
 
   syslog {
-    auth {
-      default = 0
-    }
     daemon {
       tnc = 3
       imc = 2