Drop busname unit type

Since busname units are only useful with kdbus, they weren't actively
used. This was dead code, only compile-tested. If busname units are
ever added back, it'll be cleaner to start from scratch (possibly reverting
parts of this patch).

src/systemctl/systemctl.c

@@ -7258,11 +7258,6 @@ static void help_states(void) {
         for (i = 0; i < _AUTOMOUNT_STATE_MAX; i++)
                 puts(automount_state_to_string(i));
 
-        if (!arg_no_legend)
-                puts("\nAvailable busname unit substates:");
-        for (i = 0; i < _BUSNAME_STATE_MAX; i++)
-                puts(busname_state_to_string(i));
-
         if (!arg_no_legend)
                 puts("\nAvailable device unit substates:");
         for (i = 0; i < _DEVICE_STATE_MAX; i++)