selftests/xsk: remove namespaces

Remove the namespaces used as they fill no function. This will
simplify the code for speeding up the tests in the following commits.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230111093526.11682-9-magnus.karlsson@gmail.com


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