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>
