Remove support for TokenType.User (#958)

* Set usage of TokenType.User as an error rather than a warning.

* Remove commented sections and #pragma's

Additionally, changes use of ReadMessages to ViewChannel since that Obsolete was also suppressed by the pragma

src/Discord.Net.Commands/Attributes/Preconditions/RequireOwnerAttribute.cs

@@ -1,4 +1,3 @@
-#pragma warning disable CS0618
 using System;
 using System.Threading.Tasks;
 
@@ -20,10 +19,6 @@ namespace Discord.Commands
                     if (context.User.Id != application.Owner.Id)
                         return PreconditionResult.FromError("Command can only be run by the owner of the bot");
                     return PreconditionResult.FromSuccess();
-                case TokenType.User:
-                    if (context.User.Id != context.Client.CurrentUser.Id)
-                        return PreconditionResult.FromError("Command can only be run by the owner of the bot");
-                    return PreconditionResult.FromSuccess();
                 default:
                     return PreconditionResult.FromError($"{nameof(RequireOwnerAttribute)} is not supported by this {nameof(TokenType)}.");                    
             }