ALSA: emu10k1: remove unused emu->pcm_playback_efx_substream field

Amends historic commit 27ae958cf6 ("emu10k1 driver - add multichannel
device hw:x,3 [2-8/8]").

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Link: https://lore.kernel.org/r/20230422132430.1057468-2-oswald.buddenhagen@gmx.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>