selftests/xsk: remove unused variable outstanding_tx

Remove the unused variable outstanding_tx.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230111093526.11682-6-magnus.karlsson@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>