s390/bpf: Remove unused SEEN_RET0, SEEN_REG_AX and ret0_ip

We don't need them since commit e1cf4bef ("bpf, s390x: remove
ld_abs/ld_ind") and commit a3212b8f ("bpf, s390x: remove obsolete
exception handling from div/mod").

Also, use BIT(n) instead of 1 << n, because checkpatch says so.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191107114033.90505-1-iii@linux.ibm.com