ALSA: spi: Drop NULL check for snd_ctl_remove()

Since snd_ctl_remove() accepts the NULL kcontrol argument now, we can
drop the check in the caller side.

Link: https://lore.kernel.org/20240617100529.6667-6-tiwai@suse.de


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