ALSA: seq: Remove superfluous snd_seq_queue_client_leave_cells() call

With the previous two fixes for the write / ioctl races:
  ALSA: seq: Don't allow resizing pool in use
  ALSA: seq: More protection for concurrent write and ioctl races
the cells aren't any longer in queues at the point calling
snd_seq_pool_done() in snd_seq_ioctl_set_client_pool().  Hence the
function call snd_seq_queue_client_leave_cells() can be dropped safely
from there.

Suggested-by: Nicolai Stange <nstange@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>