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-analyze

@@ -31,7 +31,7 @@ __get_machines() {
 }
 
 __get_services() {
-    systemctl list-units --no-legend --no-pager -t service --all $1 | \
+    systemctl list-units --no-legend --no-pager --plain -t service --all $1 | \
         { while read -r a b c; do [[ $b == "loaded" ]]; echo " $a"; done }
 }