net: remove the unnecessary check in cipso_v4_doi_free

The commit 733c99ee ("net: fix NULL pointer reference in
cipso_v4_doi_free") was merged by a mistake, this patch try
to cleanup the mess.

And we already have the commit e842cb60

 ("net: fix NULL
pointer reference in cipso_v4_doi_free") which fixed the root
cause of the issue mentioned in it's description.

Suggested-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Michael Wang <yun.wang@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>