Remove first_message field as it's now unused.

track.c

@@ -93,9 +93,6 @@ struct aircraft *trackCreateAircraft(struct modesMessage *mm) {
     // don't immediately emit, let some data build up
     a->fatsv_last_emitted = a->fatsv_last_force_emit = messageNow();
 
-    // Copy the first message so we can emit it later when a second message arrives.
-    a->first_message = *mm;
-
     // initialize data validity ages
 #define F(f,s,e) do { a->f##_valid.stale_interval = (s) * 1000; a->f##_valid.expire_interval = (e) * 1000; } while (0)
     F(callsign,        60, 70);  // ADS-B or Comm-B

track.h

@@ -266,8 +266,6 @@ struct aircraft {
     uint64_t      fatsv_last_force_emit;          // time (millis) we last emitted only-on-change data
 
     struct aircraft *next;        // Next aircraft in our linked list
-
-    struct modesMessage first_message;  // A copy of the first message we received for this aircraft.
 };
 
 /* Mode A/C tracking is done separately, not via the aircraft list,