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/bind/testdata/structs.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 structs
-
-type S struct {
-        X, Y       float64
-        unexported bool
-}
-
-func (s *S) Sum() float64 {
-        return s.X + s.Y
-}