selftests: xsk: Remove unused function

Probably it was ported from xdpsock but is not used anywhere.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210329224316.17793-4-maciej.fijalkowski@intel.com