xfrm: remove stray synchronize_rcu from xfrm_init

This function is called during boot, from ipv4 stack, there is no need
to set the pointer to NULL (static storage duration, so already NULL).

No need for the synchronize_rcu either.  Remove both.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>