Remove the no longer used kitty-echo private DCS escape code

kitty/vt-parser.c

@@ -662,7 +662,6 @@ parse_kitty_dcs(PS *self, uint8_t *buf, size_t bufsz) {
     dispatch("overlay-ready|", handle_overlay_ready, 0)
     dispatch("kitten-result|", handle_kitten_result, 0)
     dispatch("print|", handle_remote_print, 0)
-    dispatch("echo|", handle_remote_echo, 0)
     dispatch("ssh|", handle_remote_ssh, 0)
     dispatch("ask|", handle_remote_askpass, 0)
     dispatch("clone|", handle_remote_clone, 0)