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

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

Index: go/src/infra/libs/clock/clock.go
diff --git a/go/src/infra/libs/clock/clock.go b/go/src/infra/libs/clock/clock.go
deleted file mode 100644
index 68b6fd24d8de1a20f68b857d1b7046d69eb8de7f..0000000000000000000000000000000000000000
--- a/go/src/infra/libs/clock/clock.go
+++ /dev/null
@@ -1,21 +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"
-)
-
-// 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.
-}