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/sprite/clock/clock.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 clock provides a clock and time functions for a sprite engine.
-package clock // import "golang.org/x/mobile/sprite/clock"
-
-// A Time represents an instant in sprite time.
-//
-// The application using the sprite engine is responsible for
-// determining sprite time.
-//
-// Typically time 0 is when the app is initialized and time is
-// quantized at the intended frame rate. For example, an app may
-// record wall time when it is initialized
-//
-//      var start = time.Now()
-//
-// and then compute the current instant in time for 60 FPS:
-//
-//      now := clock.Time(time.Since(start) * 60 / time.Second)
-//
-// An application can pause or reset sprite time, but it must be aware
-// of any stateful sprite.Arranger instances that expect time to
-// continue.
-type Time int32