selftests: xsk: cleanup bash scripts

Remove the spec-file that is not used any longer from the shell
scripts. Also remove an unused option.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20220510115604.8717-2-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>