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/bind/java/doc.go

-// Copyright 2015 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.
-
-// Package java implements the Java language bindings.
-//
-// See the design document (http://golang.org/s/gobind).
-//
-// Currently, this works only for android.
-package java
-
-// Init initializes communication with Java.
-// Typically called from the Start callback in app.Run.
-func Init() {
-        initSeq()
-}