PPP: remove useless ppp_link_start function

This function only set PPP to initialize phase, and it is only called at
the very beginning of functions where it is called. It means we could
as well set the initialize phase before calling those functions in the
PPP core.

src/include/netif/ppp/ppp_impl.h

@@ -394,9 +394,6 @@ int ppp_init(void);
 ppp_pcb *ppp_new(struct netif *pppif, const struct link_callbacks *callbacks, void *link_ctx_cb,
                  ppp_link_status_cb_fn link_status_cb, void *ctx_cb);
 
-/* Called when link is starting */
-void ppp_link_start(ppp_pcb *pcb);
-
 /* Initiate LCP open request */
 void ppp_start(ppp_pcb *pcb);