ALSA: opl3: remove redundant pointer opl3

Variable opl3 is being assigned but is never used hence it is
redundant and can be removed.

Cleans up several clang warnings:
warning: variable 'opl3' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>