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.

src/client/Utils.ts

@@ -516,7 +516,6 @@ export function getMessageTypeClasses(type: MessageType): string {
     case MessageType.ATTACK_FAILED:
     case MessageType.ALLIANCE_REJECTED:
     case MessageType.ALLIANCE_BROKEN:
-    case MessageType.UNIT_CAPTURED_BY_ENEMY:
     case MessageType.UNIT_DESTROYED:
     case MessageType.NUKE_DETONATED:
       return severityColors["fail"];