[NFCI] Remove unused variables and enable -Wunused-variable (#1483)

Remove unused variables throughout the code base and enable the
`-Wunused-variable` warning flag globally to prevent new unused
variable issues being introduced in the future.

This is mostly a non-functional change, with one exception:

 - In `test_conformance/api/test_kernel_arg_info.cpp`, an error check
   of the clGetDeviceInfo return value was added.

Signed-off-by: Sven van Haastregt <sven.vanhaastregt@arm.com>

test_conformance/printf/util_printf.cpp

@@ -842,8 +842,6 @@ static void hexRefBuilder(printDataGenParameters& params, char* refResult, const
 */
 void generateRef(const cl_device_id device)
 {
-    int fd = -1;
-    char _refBuffer[ANALYSIS_BUFFER_SIZE];
     const cl_device_fp_config fpConfig = get_default_rounding_mode(device);
     const RoundingMode hostRound = get_round();
     RoundingMode deviceRound;