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_test.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.
-
-// +build linux darwin
-
-package font
-
-import (
-        "testing"
-
-        "code.google.com/p/freetype-go/freetype"
-)
-
-func TestLoadFonts(t *testing.T) {
-        if _, err := freetype.ParseFont(Default()); err != nil {
-                t.Fatalf("default font: %v", err)
-        }
-        if _, err := freetype.ParseFont(Monospace()); err != nil {
-                t.Fatalf("monospace font: %v", err)
-        }
-}