audio: remove a duplicate clearing of a struct variable

Fixes #8301

player/audio.c

@@ -218,7 +218,6 @@ static void ao_chain_reset_state(struct ao_chain *ao_c)
 {
     ao_c->last_out_pts = MP_NOPTS_VALUE;
     ao_c->out_eof = false;
-    ao_c->underrun = false;
     ao_c->start_pts_known = false;
     ao_c->start_pts = MP_NOPTS_VALUE;
     ao_c->untimed_throttle = false;