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

go/src/infra/libs/clock/systemclock.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"
-)
-
-// Implementation of Clock that uses Go's standard library.
-type systemClock struct{}
-
-// System clock instance.
-var systemClockInstance systemClock
-
-var _ Clock = systemClock{}
-
-// GetSystemClock returns an instance of a Clock whose method calls directly use
-// Go's "time" library.
-func GetSystemClock() Clock {
-        return systemClockInstance
-}
-
-func (systemClock) Now() time.Time {
-        return time.Now()
-}
-
-func (systemClock) Sleep(d time.Duration) {
-        time.Sleep(d)
-}
-
-func (systemClock) NewTimer() Timer {
-        return new(systemTimer)
-}
-
-func (systemClock) After(d time.Duration) <-chan time.Time {
-        return time.After(d)
-}