Remove unused mutex from Emotes (#3943)

The mutex was initially used to limit access to the twitchEmotesCache_ member
but it's no longer necessary since it's been made a UniqueAccess type

src/providers/twitch/TwitchEmotes.hpp

@@ -45,8 +45,6 @@ private:
     Url getEmoteLink(const EmoteId &id, const QString &emoteScale);
     UniqueAccess<std::unordered_map<EmoteId, std::weak_ptr<Emote>>>
         twitchEmotesCache_;
-
-    std::mutex mutex_;
 };
 
 }  // namespace chatterino