net/devif/devif_callback.c: Remove a redundant assignment

Found by clang-check:

devif/devif_callback.c:508:7: warning: Value stored to 'cb' is never read
      cb = next;
      ^    ~~~~
1 warning generated.

net/devif/devif_callback.c

@@ -502,10 +502,6 @@ uint16_t devif_dev_event(FAR struct net_driver_s *dev, void *pvconn,
 
           flags = cb->event(dev, pvconn, cb->priv, flags);
         }
-
-      /* Set up for the next time through the loop */
-
-      cb = next;
     }
 
   net_unlock();