ao_null: disable latency emulation

Doesn't work quite right, and will pause for the latency duration after
seeking. Some users use --ao=null to disable audio (even though they
should probably use --no-audio), and this use-case is broken by this
issue too.

CC: @mpv-player/stable

audio/out/ao_null.c

@@ -210,7 +210,6 @@ const struct ao_driver audio_out_null = {
     .priv_size = sizeof(struct priv),
     .priv_defaults = &(const struct priv) {
         .bufferlen = 0.2,
-        .latency_sec = 0.5,
         .outburst = 256,
         .speed = 1,
     },