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/interfaces.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 interfaces
-
-type I interface {
-        Rand() int32
-}
-
-func Add3(r I) int32 {
-        return r.Rand() + r.Rand() + r.Rand()
-}
-
-// chosen by fair dice roll.
-// guaranteed to be random.
-type seven struct{}
-
-func (seven) Rand() int32 { return 7 }
-
-func Seven() I { return seven{} }