pid1: drop kdbus_fd and all associated logic

src/core/manager.h

@@ -294,9 +294,6 @@ struct Manager {
          * value where Unit objects are contained. */
         Hashmap *units_requiring_mounts_for;
 
-        /* Reference to the kdbus bus control fd */
-        int kdbus_fd;
-
         /* Used for processing polkit authorization responses */
         Hashmap *polkit_registry;