Implement IMessageChannel#DeleteMessageAsync (#996)

* Implement DeleteMessageAsync

* Refer to MessageHelper instead of duplicating call

* Fix refactor error

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -185,6 +185,10 @@ namespace Discord.Rest
             return RestUserMessage.Create(client, channel, client.CurrentUser, model);
         }
 
+        public static Task DeleteMessageAsync(IMessageChannel channel, ulong messageId, BaseDiscordClient client,
+            RequestOptions options)
+            => MessageHelper.DeleteAsync(channel.Id, messageId, client, options);
+
         public static async Task DeleteMessagesAsync(ITextChannel channel, BaseDiscordClient client,
             IEnumerable<ulong> messageIds, RequestOptions options)
         {