mfd: ab8500-gpadc: Remove unused 'struct ab8500_gpadc'

The ab8500_gpadc structure carries lots of important information which
most functions make good use of. The initial expectation was that
ab8500_gpadc_runtime_idle() would be no exception; however, this hasn't
been the case to date. Let's remove it for now and add it back in only
when we have a use for it.
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/ab8500-gpadc.c

@@ -601,8 +601,6 @@ static int ab8500_gpadc_runtime_resume(struct device *dev)
 
 static int ab8500_gpadc_runtime_idle(struct device *dev)
 {
-	struct ab8500_gpadc *gpadc = dev_get_drvdata(dev);
-
 	pm_runtime_suspend(dev);
 	return 0;
 }