systemctl: hide first column with --plain instead of --no-legend

Hiding the first column, which may contain bullet circles, with --no-legend
is undocumented and potentially unexpected. On the other hand, not printing
bullet circles with --plain is documented so hiding the column with that
switch is sensible.

The combination "--full --no-legend --no-pager --plain" is appropriate for
automated processing of systemctl output.

shell-completion/bash/systemd-cgls

@@ -29,7 +29,7 @@ __get_machines() {
 }
 
 __get_units_have_cgroup() {
-    systemctl $1 list-units | {
+    systemctl $1 --full --no-legend --no-pager --plain list-units | {
         while read -r a b c d; do
             [[ $c == "active" && ${a##*.} =~ (service|socket|mount|swap|slice|scope) ]] && echo " $a"
         done