PPP, remove useless ppp_init function

Simon says:

ppp_init() does not seem to be used. The only thing it does is calling
magic_init(), which is not required because it its called again later
from ppp_input().

Also, the time from startup is rather constant, so calling sys_jiffies()
from ppp_init() does not create a random number.

src/core/init.c

@@ -59,7 +59,6 @@
 #include "lwip/nd6.h"
 #include "lwip/mld6.h"
 #include "lwip/api.h"
-#include "netif/ppp/ppp_impl.h"
 
 /* Compile-time sanity checks for configuration errors.
  * These can be done independently of LWIP_DEBUG, without penalty.
@@ -345,9 +344,6 @@ lwip_init(void)
 #if LWIP_DNS
   dns_init();
 #endif /* LWIP_DNS */
-#if PPP_SUPPORT
-  ppp_init();
-#endif
 
 #if LWIP_TIMERS
   sys_timeouts_init();