selftests/xsk: remove unnecessary code in control path

selftests/xsk: remove unnecessary code in control path

Remove unnecessary code in the control path. This is located in the
file xsk.c that was moved from libbpf when the xsk support there was
deprecated. Some of the code there is not needed anymore as the
selftests are only guaranteed to run on the kernel it is shipped
with. Therefore, all the code that has to deal with compatibility of
older kernels can be dropped and also any other function that is not
of any use for the tests.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/r/20230111093526.11682-11-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>