[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/compiler/test_compiler_defines_for_extensions.cpp

@@ -361,8 +361,6 @@ int test_compiler_defines_for_extensions(cl_device_id device, cl_context context
     clProgramWrapper program;
     clKernelWrapper kernel;
 
-    Version version = get_device_cl_version(device);
-
     error = create_single_kernel_helper(context, &program, &kernel, 1,
                                         (const char **)&kernel_code, "test");
     test_error(error, "create_single_kernel_helper failed");