[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/api/test_null_buffer_arg.cpp

@@ -149,7 +149,6 @@ int test_null_buffer_arg(cl_device_id device, cl_context context,
     cl_command_queue queue, int num_elements)
 {
     unsigned int test_success = 0;
-    unsigned int i;
     unsigned int buffer_size;
     cl_int status;
     cl_program program;