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/README.md

-# Go support for Mobile devices
-
-The Go mobile repository holds packages and build tools for using Go on Android.
-
-This is early work and the build system is a bumpy ride. Building a binary for
-Android requires using a Go cross compiler and an external linker from the NDK.
-
-For now, the easiest way to setup a build environment is using the provided
-Dockerfile:
-
-        docker pull golang/mobile
-
-Get the sample applications.
-
-        go get -d golang.org/x/mobile/example/...
-
-In your app directory under your `$GOPATH`, copy the following files from either
-the [golang.org/x/mobile/example/basic](https://github.com/golang/mobile/tree/master/example/basic)
-or [golang.org/x/mobile/example/libhello](https://github.com/golang/mobile/tree/master/example/libhello)
-apps:
-
-        AndroidManifest.xml
-        all.bash
-        build.xml
-        jni/Android.mk
-        make.bash
-
-Start with `basic` if you are writing an all-Go application (that is, an OpenGL game)
-or libhello if you are building a `.so` file for use from Java via
-[gobind](https://godoc.org/golang.org/x/mobile/cmd/gobind). Edit the files to change
-the name of your app.
-
-To build, run:
-
-        docker run -v $GOPATH/src:/src golang/mobile /bin/bash -c 'cd /src/your/project && ./make.bash'
-
-Note the use of -v option to mount $GOPATH/src to /src of the container.
-The above command will fail if the -v option is missing or the specified
-volume is not accessible from the container.
-
-When working with an all-Go application, this will produce a binary at
-`$GOPATH/src/your/project/bin/name-debug.apk`. You can use the adb tool to install
-and run this app. See all.bash for an example.
-
---
-
-APIs are currently very limited, but under active development. Package
-documentation serves as a starting point:
-
-- [mobile/app](http://godoc.org/golang.org/x/mobile/app)
-- [mobile/gl](http://godoc.org/golang.org/x/mobile/gl)
-- [mobile/sprite](http://godoc.org/golang.org/x/mobile/sprite)
-- [mobile/cmd/gobind](http://godoc.org/golang.org/x/mobile/cmd/gobind)
-
-Contributions to Go are appreciated. See https://golang.org/doc/contribute.html.