Remove stale text from comment.

Comment for CurrentRoute claimed that setting the KeepContext option would propagate
the Route even after the request. The KeepContext option is deprecated and has no effect.

mux.go

@@ -435,8 +435,7 @@ func Vars(r *http.Request) map[string]string {
 // CurrentRoute returns the matched route for the current request, if any.
 // This only works when called inside the handler of the matched route
 // because the matched route is stored in the request context which is cleared
-// after the handler returns, unless the KeepContext option is set on the
-// Router.
+// after the handler returns.
 func CurrentRoute(r *http.Request) *Route {
 	if rv := r.Context().Value(routeKey); rv != nil {
 		return rv.(*Route)