ALSA: usb-audio: Remove unused function declaration

Commit 68e67f40 ("ALSA: snd-usb: move calls to usb_set_interface")
leave this unused declaration.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20230801144512.18716-1-yuehaibing@huawei.com


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