CLEANUP: connection: use fd_stop_both() instead of conn_stop_polling()

conn_stop_polling() in fact only calls fd_stop_both() after checking
that the ctrl layer is ready. It's the case in conn_fd_check() so
let's get rid of this next-to-last user of this function.
diff --git a/src/connection.c b/src/connection.c
index 366bb25..13c9930 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -264,7 +264,7 @@
 	 */
 	fdtab[fd].linger_risk = 0;
 	conn->flags |= CO_FL_ERROR | CO_FL_SOCK_RD_SH | CO_FL_SOCK_WR_SH;
-	conn_stop_polling(conn);
+	fd_stop_both(fd);
 	return 0;
 
  wait: