example_app/lwipopts: don't define PBUF_LINK_HLEN to 16

This breaks PBUF_RAW against pbuf_add_header(PBUF_LINK_HLEN) like used
at least in icmp. Redefine ETH_PAD_SIZE instead if this is required.

Signed-off-by: Simon Goldschmidt <goldsimon@gmx.de>

examples/example_app/lwipopts.h

@@ -173,10 +173,6 @@ a lot of data that needs to be copied, this should be set high. */
 /* PBUF_POOL_BUFSIZE: the size of each pbuf in the pbuf pool. */
 #define PBUF_POOL_BUFSIZE       128
 
-/* PBUF_LINK_HLEN: the number of bytes that should be allocated for a
-   link level header. */
-#define PBUF_LINK_HLEN          16
-
 /** SYS_LIGHTWEIGHT_PROT
  * define SYS_LIGHTWEIGHT_PROT in lwipopts.h if you want inter-task protection
  * for certain critical regions during buffer allocation, deallocation and memory