MINOR: servers: Kill priv_conns.

Remove the list of private connections from server, it has been largely
unused, we only inserted connections in it, but we would never actually
use it.
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index 7254add..02a5e2c 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -3503,9 +3503,7 @@
 			struct server *srv = objt_server(fconn->conn->target);
 
 			if (srv) {
-				if (fconn->conn->flags & CO_FL_PRIVATE)
-					LIST_ADD(&srv->priv_conns[tid], &fconn->conn->list);
-				else
+				if (!(fconn->conn->flags & CO_FL_PRIVATE))
 					LIST_ADD(&srv->idle_conns[tid], &fconn->conn->list);
 			}
 			TRACE_DEVEL("connection in idle server list", FCGI_EV_STRM_END, fconn->conn);