openvswitch: Revert: "Enable memory mapped Netlink i/o"

revert commit 795449d8 ("openvswitch: Enable memory mapped Netlink i/o").
Following the mmaped netlink removal this code can be removed.

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