Fix mistake: Use IMessageCache for SocketGroupChannel

src/Discord.Net.WebSocket/Entities/Channels/SocketGroupChannel.cs

@@ -38,8 +38,7 @@ namespace Discord.WebSocket
         internal SocketGroupChannel(DiscordSocketClient discord, ulong id)
             : base(discord, id)
         {
-            if (Discord.MessageCacheSize > 0)
-                _messages = new MessageCache(Discord);
+            _messages = discord.MessageCache.CreateMessageCache(discord);
             _voiceStates = new ConcurrentDictionary<ulong, SocketVoiceState>(ConcurrentHashSet.DefaultConcurrencyLevel, 5);
             _users = new ConcurrentDictionary<ulong, SocketGroupUser>(ConcurrentHashSet.DefaultConcurrencyLevel, 5);
         }

src/Discord.Net.WebSocket/Entities/Messages/MessageCache.cs

@@ -14,10 +14,7 @@ namespace Discord.WebSocket
 
         public IReadOnlyCollection<SocketMessage> Messages => _messages.ToReadOnlyCollection();
 
-        public IMessageCache CreateMessageCache(DiscordSocketClient discord)
-        {
-            return new MessageCache(discord);
-        }
+        public IMessageCache CreateMessageCache(DiscordSocketClient discord) => new MessageCache(discord);
 
         public MessageCache(DiscordSocketClient discord)
         {