nfnetlink: Revert "nfnetlink: add support for memory mapped netlink"

reverts commit 3ab1f683 ("nfnetlink: add support for memory mapped
netlink")'

Like previous commits in the series, remove wrappers that are not needed
after mmapped netlink removal.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>