don't throw in CommandLineBuilder ctor

Author: jonsequitur

src/System.CommandLine/Builder/CommandLineBuilder.cs

@@ -17,10 +17,6 @@ public class CommandLineBuilder : CommandBuilder
         public CommandLineBuilder(Command rootCommand = null)
             : base(rootCommand ?? new RootCommand())
         {
-            if (rootCommand?.ImplicitParser != null)
-            {
-                throw new ArgumentException($"Command \"{rootCommand.Name}\" has already been configured.");
-            }
         }
 
         public bool EnableDirectives { get; set; } = true;

src/System.CommandLine/Command.cs

@@ -3,7 +3,6 @@
 
 using System.Collections;
 using System.Collections.Generic;
-using System.CommandLine.Builder;
 using System.CommandLine.Collections;
 using System.CommandLine.Invocation;
 using System.CommandLine.Parsing;