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/include/netif/ppp/ppp_impl.h

@@ -378,13 +378,6 @@ struct pppd_stats {
  * PPP private functions
  */
 
-/*
- * Functions called from lwIP core.
- */
-
-/* initialize the PPP subsystem */
-int ppp_init(void);
-
 
 /*
  * Functions called from PPP link protocols.