Remove duplicate types:
* ebpf_helper_return_type_t
* ebpf_helper_argument_type_t
* ebpf_context_descriptor_t
https://github.com/vbpf/ebpf-verifier/pull/238 already made the
necessary changes in the PREVAIL project, which got rid of the
issue with VOID and so the 'undef VOID' workarounds are removed
in the present PR.
Signed-off-by: Dave Thaler <dthaler@ntdev.microsoft.com>