Fix briefing generation.

I removed the nav target info from the briefing because that doesn't
seem to have been doing what it was intended to do. It didn't give any
actual target information, all it would show was (example is a  JF-17
strike mission):

    PP1
    PP2
    PP3
    PP4

Without any additional context that doesn't seem too helpful to me.
I'll be following up (hopefully) shortly by adding target information
(type, coordinates, STPT/PP, etc) to both the briefing and the
kneeboard that will cover that.

Refactor a bunch to share some code with the kneeboard generator as
well.

game/operation/frontlineattack.py

@@ -36,6 +36,4 @@ class FrontlineAttackOperation(Operation):
     def generate(self):
         self.briefinggen.title = "Frontline CAS"
         self.briefinggen.description = "Provide CAS for the ground forces attacking enemy lines. Operation will be considered successful if total number of enemy units will be lower than your own by a factor of 1.5 (i.e. with 12 units from both sides, enemy forces need to be reduced to at least 8), meaning that you (and, probably, your wingmans) should concentrate on destroying the enemy units. Target base strength will be lowered as a result. Be advised that your flight will not attack anything until you explicitly tell them so by comms menu."
-        self.briefinggen.append_waypoint("CAS AREA IP")
-        self.briefinggen.append_waypoint("CAS AREA EGRESS")
         super(FrontlineAttackOperation, self).generate()