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/example/libhello/main.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.
-
-// This is the Go entry point for the libhello app.
-// It is invoked from Java.
-//
-// See README for details.
-package main
-
-import (
-        "golang.org/x/mobile/app"
-
-        "golang.org/x/mobile/bind/java"
-        _ "golang.org/x/mobile/example/libhello/hi/go_hi"
-)
-
-func main() {
-        app.Run(app.Callbacks{Start: java.Init})
-}