remove deprecated pkg/locker

This package was deprecated in 5ca758199d77a66f7186450f2e78b0a915d443c4, which
was part of the 20.10 release, so consumers of this package should've been
able to migrate to the new location.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/locker/locker.go

@@ -1,30 +0,0 @@
-/*
-Package locker provides a mechanism for creating finer-grained locking to help
-free up more global locks to handle other tasks.
-
-The implementation looks close to a sync.Mutex, however the user must provide a
-reference to use to refer to the underlying lock when locking and unlocking,
-and unlock may generate an error.
-
-If a lock with a given name does not exist when `Lock` is called, one is
-created.
-Lock references are automatically cleaned up on `Unlock` if nothing else is
-waiting for the lock.
-*/
-package locker // import "github.com/docker/docker/pkg/locker"
-
-import (
-	"github.com/moby/locker"
-)
-
-// ErrNoSuchLock is returned when the requested lock does not exist
-// Deprecated: use github.com/moby/locker.ErrNoSuchLock
-var ErrNoSuchLock = locker.ErrNoSuchLock
-
-// Locker provides a locking mechanism based on the passed in reference name
-// Deprecated: use github.com/moby/locker.Locker
-type Locker = locker.Locker
-
-// New creates a new Locker
-// Deprecated: use github.com/moby/locker.New
-var New = locker.New