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

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

Index: go/src/infra/libs/clock/systemclock.go
diff --git a/go/src/infra/libs/clock/systemclock.go b/go/src/infra/libs/clock/systemclock.go
deleted file mode 100644
index b5384a411fed547f99b9067c52abd09f2d2b38b6..0000000000000000000000000000000000000000
--- a/go/src/infra/libs/clock/systemclock.go
+++ /dev/null
@@ -1,39 +0,0 @@
-// 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)
-}