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_h2.c b/src/mux_h2.c
index b872897..013ef86 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -3901,9 +3901,7 @@
 				struct server *srv = objt_server(h2c->conn->target);
 
 				if (srv) {
-					if (h2c->conn->flags & CO_FL_PRIVATE)
-						LIST_ADD(&srv->priv_conns[tid], &h2c->conn->list);
-					else
+					if (!(h2c->conn->flags & CO_FL_PRIVATE))
 						LIST_ADD(&srv->idle_conns[tid], &h2c->conn->list);
 				}