Remove auth_challenge field from or_handshake_state_t

We didn't need to record this value; it was already recorded
implicitly while computing cell digests for later examination in the
authenticate cells.

src/or/command.c

@@ -1043,9 +1043,6 @@ command_process_auth_challenge_cell(var_cell_t *cell, or_connection_t *conn)
   if (cell->payload_len < OR_AUTH_CHALLENGE_LEN + 2 + 2*n_types)
     ERR("It looks truncated");
 
-  memcpy(conn->handshake_state->auth_challenge, cell->payload,
-         OR_AUTH_CHALLENGE_LEN);
-
   /* Now see if there is an authentication type we can use */
   cp=cell->payload+OR_AUTH_CHALLENGE_LEN+2;
   for (i=0; i < n_types; ++i, cp += 2) {

src/or/or.h

@@ -1162,9 +1162,6 @@ typedef struct or_handshake_state_t {
    * on this connection. */
   uint8_t authenticated_peer_id[DIGEST_LEN];
 
-  /** DOCDOC  */
-  uint8_t auth_challenge[OR_AUTH_CHALLENGE_LEN];
-
   /** Digests of the cells that we have sent or received as part of a V3
    * handshake.  Used for making and checking AUTHENTICATE cells.
    *