cleanup: Remove ChannelHelper#IsNsfw

Closes #1074

This code was not referenced anywhere in the active codebase.

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

@@ -337,12 +337,5 @@ namespace Discord.Rest
                 author = RestUser.Create(client, guild, model, webhookId);
             return author;
         }
-
-        [Obsolete("Use channel.IsNsfw instead")]
-        public static bool IsNsfw(IChannel channel)
-            => IsNsfw(channel.Name);
-        [Obsolete("Use Channel.IsNsfw instead")]
-        public static bool IsNsfw(string channelName) =>
-            channelName == "nsfw" || channelName.StartsWith("nsfw-");
     }
 }