Remove DEFCON 2 warning when Nuke Subs is in effect.

[Tuch-o]

Quoting JAN:

Minor nitpick- It's a bit irritating to see the DEFCON message when Nuke Subs is in effect as the US- Yes, I know DEFCON is 2, yes, it is a perfectly fine maneuver. Nuke Subs is in effect.