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/gl/doc.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.
-
-/*
-Package gl implements Go bindings for OpenGL ES 2.
-
-The bindings are deliberately minimal, staying as close the C API as
-possible. The semantics of each function maps onto functions
-described in the Khronos documentation:
-
-https://www.khronos.org/opengles/sdk/docs/man/
-
-One notable departure from the C API is the introduction of types
-to represent common uses of GLint: Texture, Surface, Buffer, etc.
-
-A tracing version of the OpenGL bindings is behind the `gldebug` build
-tag. It acts as a simplified version of apitrace. Build your Go binary
-with
-
-        -tags gldebug
-
-and each call to a GL function will log its input, output, and any
-error messages. For example,
-
-        I/GoLog   (27668): gl.GenBuffers(1) [Buffer(70001)]
-        I/GoLog   (27668): gl.BindBuffer(ARRAY_BUFFER, Buffer(70001))
-        I/GoLog   (27668): gl.BufferData(ARRAY_BUFFER, 36, len(36), STATIC_DRAW)
-        I/GoLog   (27668): gl.BindBuffer(ARRAY_BUFFER, Buffer(70001))
-        I/GoLog   (27668): gl.VertexAttribPointer(Attrib(0), 6, FLOAT, false, 0, 0)  error: [INVALID_VALUE]
-
-The gldebug tracing has very high overhead, so make sure to remove
-the build tag before deploying any binaries.
-*/
-package gl // import "golang.org/x/mobile/gl"
-
-//go:generate go run gendebug.go -o gldebug.go