Remove unit capture event messages

Drop the "Your {unit} was captured by {name}" and "Captured {unit} from
{name}" display messages on unit ownership change in UnitImpl. They fired
on every capture — dominated by warships taking trade ships — and were
too spammy to be useful, so players tuned them out.

Also clean up the now-unused pieces:
- Remove the UNIT_CAPTURED_BY_ENEMY message type, its category mapping,
  and its case in getMessageTypeClasses.
- Remove the orphaned unit_captured_by_enemy and captured_enemy_unit
  en.json keys.

CAPTURED_ENEMY_UNIT is kept — still used by the trade-ship gold message.

resources/lang/en.json

@@ -1135,8 +1135,6 @@
     "received_troops_from_player": "Received {troops} troops from {name}",
     "sent_gold_to_player": "Sent {gold} gold to {name}",
     "received_gold_from_player": "Received {gold} gold from {name}",
-    "unit_captured_by_enemy": "Your {unit} was captured by {name}",
-    "captured_enemy_unit": "Captured {unit} from {name}",
     "unit_destroyed": "Your {unit} was destroyed",
     "no_boats_available": "No boats available, max {max}"
   },