xfrm: remove the unnecessary .net_exit for xfrmi

The xfrm_if(s) on each netns can be deleted when its xfrmi dev is
deleted. xfrmi dev's removal can happen when:

  a. netns is being removed and all xfrmi devs will be deleted.

  b. rtnl_link_unregister(&xfrmi_link_ops) in xfrmi_fini() when
     xfrm_interface.ko is being unloaded.

So there's no need to use xfrmi_exit_net() to clean any xfrm_if up.

v1->v2:
  - Fix some changelog.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>