Remove libs/clock in favor of luci-go/common/clock.

go/src/infra/libs/clock/clock.go

-// Copyright 2014 The Chromium 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
-
-import (
-        "time"
-)
-
-// A Clock is an interface to system time.
-//
-// The standard clock is SystemClock, which falls through to the system time
-// library. Another clock, FakeClock, is available to simulate time facilities
-// for testing.
-type Clock interface {
-        Now() time.Time                       // Returns the current time (see time.Now).
-        Sleep(time.Duration)                  // Sleeps the current goroutine (see time.Sleep)
-        NewTimer() Timer                      // Creates a new Timer instance.
-        After(time.Duration) <-chan time.Time // Waits a duration, then sends the current time.
-}