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.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
-
-// Default returns the default system font.
-// The font is encoded as a TTF.
-func Default() []byte {
-        b, err := buildDefault()
-        if err != nil {
-                panic(err)
-        }
-        return b
-}
-
-// Monospace returns the default system fixed-pitch font.
-// The font is encoded as a TTF.
-func Monospace() []byte {
-        b, err := buildMonospace()
-        if err != nil {
-                panic(err)
-        }
-        return b
-}