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 2e310235 ("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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/dt3000.c

@@ -508,12 +508,11 @@ static int dt3k_ai_insn_read(struct comedi_device *dev,
 			     unsigned int *data)
 {
 	int i;
-	unsigned int chan, gain, aref;
+	unsigned int chan, gain;
 
 	chan = CR_CHAN(insn->chanspec);
 	gain = CR_RANGE(insn->chanspec);
 	/* XXX docs don't explain how to select aref */
-	aref = CR_AREF(insn->chanspec);
 
 	for (i = 0; i < insn->n; i++)
 		data[i] = dt3k_readsingle(dev, DPR_SUBSYS_AI, chan, gain);