Mark ChannelHelper.IsNsfw Obsolete

Discord no longer treats channels prefixed with 'nsfw' as NSFW,
so we no longer need to have this check.

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

@@ -338,8 +338,10 @@ namespace Discord.Rest
             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-");
     }