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/libhellojni/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 example program compiles to a gojni.so shared library, that can
-be loaded from an android application. Build it by configuring a cross
-compiler (see go.mobile/README) and then running:
-
-ANDROID_APP=/path/to/Myapp/app ./make.bash
-
-This program expects app/Go.java to be included in the Android
-project, along with a Java class named Demo defining
-
-        public static native void hello();
-
-calling hello prints "Hello, world!" to logcat.
-
-This is a very early example program that does not represent the
-intended development model for Go on Android. A language binding
-generator will follow, as will gradle build system integration.
-The result will be no make.bash, and no need to write C.
-*/
-package main
-
-import "golang.org/x/mobile/app"
-
-func main() {
-        app.Run(app.Callbacks{})
-}