CLEANUP: protocol: remove the ->drain() function

No protocol defines it anymore. The last user used to be the monitor-net
stuff that got partially broken already when the tcp_drain() function
moved to conn_sock_drain() with commit e215bba95 ("MINOR: connection:
make conn_sock_drain() work for all socket families") in 1.9-dev2.

A part of this will surely move back later when non-socket connections
arrive with QUIC but better keep the API clean and implement what's
needed in time instead.

include/haproxy/protocol-t.h

@@ -106,7 +106,6 @@ struct protocol {
 	/* functions acting on connections */
 	void (*accept)(int fd);				/* generic accept function */
 	int (*connect)(struct connection *, int flags); /* connect function if any, see below for flags values */
-	int (*drain)(int fd);                           /* indicates whether we can safely close the fd */
 
 	struct list receivers;				/* list of receivers using this protocol (under proto_lock) */
 	int nb_receivers;				/* number of receivers (under proto_lock) */

src/connection.c

@@ -420,12 +420,6 @@ int conn_sock_drain(struct connection *conn)
 	if (!fd_recv_ready(conn->handle.fd))
 		return 0;
 
-	if (conn->ctrl->drain) {
-		if (conn->ctrl->drain(conn->handle.fd) <= 0)
-			return 0;
-		goto shut;
-	}
-
 	/* no drain function defined, use the generic one */
 
 	while (turns) {