[DCCP]: calling dccp_v{4,6}_reqsk_send_ack is a BUG

This patch removes two functions, the send_ack functions of request_sock,
which are not called/used by the DCCP code. It is correct that these
functions are not called, below is a justification why calling these
functions (on a passive socket in the LISTEN/RESPOND state) would mean
a DCCP protocol violation.

A) Background: using request_sock in TCP:

net/dccp/dccp.h

@@ -131,6 +131,8 @@ extern int  dccp_retransmit_skb(struct sock *sk, struct sk_buff *skb);
 
 extern void dccp_send_ack(struct sock *sk);
 extern void dccp_send_delayed_ack(struct sock *sk);
+extern void dccp_reqsk_send_ack(struct sk_buff *sk, struct request_sock *rsk);
+
 extern void dccp_send_sync(struct sock *sk, const u64 seq,
 			   const enum dccp_pkt_type pkt_type);