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/font/font_android.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 font
-
-import "io/ioutil"
-
-func buildDefault() ([]byte, error) {
-        return ioutil.ReadFile("/system/fonts/DroidSans.ttf")
-}
-
-func buildMonospace() ([]byte, error) {
-        return ioutil.ReadFile("/system/fonts/DroidSansMono.ttf")
-}