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/ultrasonic/ultrasonic.cxx

@@ -34,7 +34,6 @@
 using namespace upm;
 
 UltraSonic::UltraSonic (uint8_t pin) {
-    mraa_result_t error = MRAA_SUCCESS;
     m_name = "UltraSonic";
 
     mraa_init();