PPP, PPPoS, removed xmit_accm, almost duplicate of out_accm

xmit_accm was meant to be a user configurable asyncmap, it was actually
broken since the introduction of ppp_new by the way we now reset the PPP
initial state, looks like no one until now is needing it anymore. If
necessary we will reintroduce this feature later properly instead of a
dirty and ugly hack into the PPP code.

src/include/netif/ppp/ppp.h

@@ -368,9 +368,7 @@ struct ppp_pcb_s {
   unsigned int                         :5; /* 5 bits of padding to round out to 16 bits */
 
 #if PPPOS_SUPPORT
-/* FIXME: there is probably one superfluous */
   ext_accm out_accm;             /* Async-Ctl-Char-Map for output. */
-  ext_accm xmit_accm;            /* extended transmit ACCM */
 #endif /* PPPOS_SUPPORT */
 
   u32_t last_xmit;               /* Time of last transmission. */