cli/config/memorystore: remove unused IsErrValueNotFound

This utility was added in 9b83d5bbf9, but
was never used. Remove the utility, and rewrite the error returned to
implement the errdefs.NotFound interface, so that it can be detected
using the errdefs.IsNotFound() utility if needed.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 3c78ac2aad)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/config/memorystore/store.go

@@ -3,7 +3,6 @@
 package memorystore
 
 import (
-	"errors"
 	"fmt"
 	"maps"
 	"os"
@@ -13,12 +12,17 @@ import (
 	"github.com/docker/cli/cli/config/types"
 )
 
-var errValueNotFound = errors.New("value not found")
+// notFoundErr is the error returned when a plugin could not be found.
+type notFoundErr string
 
-func IsErrValueNotFound(err error) bool {
-	return errors.Is(err, errValueNotFound)
+func (notFoundErr) NotFound() {}
+
+func (e notFoundErr) Error() string {
+	return string(e)
 }
 
+var errValueNotFound notFoundErr = "value not found"
+
 type Config struct {
 	lock              sync.RWMutex
 	memoryCredentials map[string]types.AuthConfig