delete duplicate header files

components/net/lwip-2.1.2/src/netif/ethernetif.c

@@ -41,8 +41,6 @@
  * 2018-11-02     MurphyZhao   port to lwIP 2.1.0
  */
 
-#include "lwip/opt.h"
-
 #include "lwip/opt.h"
 #include "lwip/debug.h"
 #include "lwip/def.h"