xfrm: delete not-needed clear to zero of encap_oa

After commit 2f479651 ("af_key: Fix heap information leak"), there is
no need to clear encap_oa again as it is already initialized to zero.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>