remove custom exception

Graph/Exceptions/InvalidVertexException.cs

@@ -1,12 +0,0 @@
-using System;
-
-namespace MoonTools.Core.Graph
-{
-    public class InvalidVertexException : Exception
-    {
-        public InvalidVertexException(
-            string format,
-            params object[] args
-        ) : base(string.Format(format, args)) { }
-    }
-}