[adc] remove dead code

The variable result_str is never used after this assignment.

components/drivers/misc/adc.c

@@ -265,7 +265,6 @@ static int adc(int argc, char **argv)
                 if(argc == 3)
                 {
                     voltage = rt_adc_voltage(adc_device, atoi(argv[2]));
-                    result_str = (result == RT_EOK) ? "success" : "failure";
                     rt_kprintf("%s channel %d voltage is %d.%03dV \n", adc_device->parent.parent.name, (rt_base_t)atoi(argv[2]), voltage / 1000, voltage % 1000);
                 }
                 else