Remove third_party/golang.org/x/mobile as it is no longer used with Go 1.5.

third_party/go/src/golang.org/x/mobile/app/desktop.go

-// Copyright 2014 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// +build linux,!android darwin
-
-package app
-
-import (
-        "os"
-        "path/filepath"
-)
-
-// Logic shared by desktop debugging implementations.
-
-func openAsset(name string) (ReadSeekCloser, error) {
-        if !filepath.IsAbs(name) {
-                name = filepath.Join("assets", name)
-        }
-        f, err := os.Open(name)
-        if err != nil {
-                return nil, err
-        }
-        return f, nil
-}