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/udevadm

@@ -38,7 +38,7 @@ __get_all_devs() {
 }
 
 __get_all_device_units() {
-    systemctl list-units -t device --full --no-legend --no-pager 2>/dev/null | \
+    systemctl list-units -t device --full --no-legend --no-pager --plain 2>/dev/null | \
         { while read -r a b; do echo "$a"; done; }
 }