nimble/ll: Fix calculating BIG scheduler start time

The same code is executed already in big_sched_set resulting in sched
offset being subtracted twice.

nimble/controller/src/ble_ll_iso_big.c

@@ -573,10 +573,6 @@ ble_ll_iso_big_event_done(struct ble_ll_iso_big *big)
  big->anchor_offset++;
  big_sched_set(big);
 
- big->sch.end_time = big->sch.start_time +
- ble_ll_tmr_u2t_up(big->sync_delay) + 1;
- big->sch.start_time -= g_ble_ll_sched_offset_ticks;
-
  /* XXX this should always succeed since we preempt anything for now */
  rc = ble_ll_sched_iso_big(&big->sch, 0, 0);
  assert(rc == 0);