selftests/bpf: Remove arena tests from DENYLIST.s390x

Now that the s390x JIT supports arena, remove the respective tests from
the denylist.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240701234304.14336-13-iii@linux.ibm.com