Wunused-variable: Fixed all unused variables in src

This commit addresses all warnings emitted from -Wunused-variable
in the C++ src.

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/ads1x15/ads1015.cxx

@@ -149,7 +149,6 @@ ADS1015::setDelay(){
 
 ADS1X15::ADSMUXMODE
 ADS1015::getMuxMode(unsigned int input) {
-    ADS1X15::ADSMUXMODE mode;
     switch (input) {
     case 0:
         return SINGLE_0;