[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/conversions/test_conversions.cpp

@@ -1666,8 +1666,6 @@ static cl_program   MakeProgram( Type outType, Type inType, SaturationMode sat,
                                         &programSource, testName, flags);
     if (error)
     {
-        char    buffer[2048] = "";
-
         vlog_error("Failed to build kernel/program.\n", error);
         clReleaseProgram(program);
         return NULL;