dccp ccid-2: Remove wrappers around sk_{reset,stop}_timer()

This removes the wrappers around the sk timer functions as it makes the code
clearer and not much is gained from using wrappers: the BUG_ON in
start_rto_timer will never trigger since that function was called only when
 * the RTO timer expired (rto_expire, and then timer_pending() is false);
 * in tx_packet_sent only if !timer_pending() (BUG_ON is redundant here);
 * previously in new_ack, after stopping the timer (timer_pending() false).

One further motive behind this patch is to replace the RTO timer with the
icsk retransmission timer, as it is already part of the DCCP socket.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>