Remove dead asserts.

I believe the purpose of these asserts was to quiet a compiler warning
in r24309, but the rest of that revision (marking fatal as a
non-returning function) should do the job.

tcpip.cc

@@ -535,9 +535,6 @@ int send_ip_packet(int sd, const struct eth_nfo *eth,
   }
 
   fatal("%s only understands IP versions 4 and 6 (got %u)", __func__, ip->ip_v);
-
-  /* This should not be reached. Just in case. */
-  assert(0);
 }