selftests: xsk: Remove color mode

Remove color mode since it does not add any value and having less code
means less maintenance which is a good thing.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-2-magnus.karlsson@gmail.com