staging: comedi: Remove set but not used variable 'aref'

staging: comedi: Remove set but not used variable 'aref'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/staging/comedi/drivers/dt3000.c: In function dt3k_ai_insn_read:
drivers/staging/comedi/drivers/dt3000.c:511:27: warning: variable aref set but not used [-Wunused-but-set-variable]

It is not used since commit 2e310235ca8f ("staging:
comedi: dt3000: rename dt3k_ai_insn()")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/1570520515-2186-7-git-send-email-zhengbin13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>