Trivia game cleanup

src/NadekoBot/data/strings/responses/responses.en-US.json

@@ -343,6 +343,7 @@
   "trivia_times_up": "Time's up! The correct answer was {0}",
   "trivia_win": "{0} guessed it and WON the game! The answer was: {1}",
   "trivia_quit": "You can stop trivia by typing {0}",
+  "trivia_ended": "Trivia game ended",
   "ttt_against_yourself": "You can't play against yourself.",
   "ttt_already_running": "TicTacToe Game is already running in this channel.",
   "ttt_a_draw": "A draw!",