openvswitch: Remove conntrack Kconfig option.

There's no particular desire to have conntrack action support in Open
vSwitch as an independently configurable bit, rather just to ensure
there is not a hard dependency. This exposed option doesn't accurately
reflect the conntrack dependency when enabled, so simplify this by
removing the option. Compile the support if NF_CONNTRACK is enabled.

Fixes: 7f8a436e

 ("openvswitch: Add conntrack action")
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>