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.

gen/groundobjectsgen.py

@@ -189,6 +189,7 @@ class GroundObjectsGenerator:
                                 "N/A",
                                 atc=atc_channel,
                                 tacan=tacan,
+                                tacan_callsign=tacan_callsign,
                                 icls=icls_channel,
                             )