Removed route extensions for .Net Core

Both `RouteCollection_Extensions` and `RouteValueDictionary_Extensions` rely on things that were heavily modified for Asp.Net Core. It's really complex to rewrite them and it will need some thinking.

Moreover, I'm not completely conviced it it still usefull, as route information can be tested in a simpler way, cf: https://github.com/aspnet/Mvc/blob/6.0.0-rc1/test/Microsoft.AspNet.Mvc.FunctionalTests/RoutingTests.cs

Author: Gimly

src/FluentAssertions.Mvc.Shared/RouteCollection_Extensions.cs

@@ -1,4 +1,5 @@
-using System;
+#if !NETSTANDARD1_6
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
@@ -19,3 +20,4 @@ public static RouteData GetRouteDataForUrl(this RouteCollection routes, string u
         }
     }
 }
+#endif

src/FluentAssertions.Mvc.Shared/RouteValueDictionary_Extensions.cs

@@ -1,4 +1,5 @@
-using System.Web.Mvc;
+#if !NETSTANDARD1_6
+using System.Web.Mvc;
 using System.Web.Routing;
 using FluentAssertions.Mvc.Fakes;
 
@@ -15,3 +16,4 @@ public static string GenerateUrl(this RouteValueDictionary routeValues, RouteCol
         }
     }
 }
+#endif