ethernet: remove CONFIG_ETHERNET

1. Hello World application shows no footprint difference before and
after this change
2. examples/ethernet/ethernet application compiles properly (can't
test with my board)

components/esp32/include/esp_event.h

@@ -159,6 +159,11 @@ esp_err_t esp_event_send(system_event_t *event);
   */
 esp_err_t esp_event_process_default(system_event_t *event);
 
+/**
+  * @brief  Install default event handlers for Ethernet interface
+  *
+  */
+void esp_event_set_default_eth_handlers();
 
 #ifdef __cplusplus
 }