cli/registry: fix "unused-receiver" linting

cli/registry/client/fetcher.go:307:7: unused-receiver: method receiver 'n' is not referenced in method's body, consider removing or renaming it as _ (revive)
    func (n *notFoundError) NotFound() {}
          ^
    cli/registry/client/endpoint.go:126:7: unused-receiver: method receiver 'th' is not referenced in method's body, consider removing or renaming it as _ (revive)
    func (th *existingTokenHandler) Scheme() string {
          ^

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

cli/registry/client/endpoint.go

@@ -123,6 +123,6 @@ func (th *existingTokenHandler) AuthorizeRequest(req *http.Request, _ map[string
 	return nil
 }
 
-func (th *existingTokenHandler) Scheme() string {
+func (*existingTokenHandler) Scheme() string {
 	return "bearer"
 }

cli/registry/client/fetcher.go

@@ -304,4 +304,4 @@ func (n *notFoundError) Error() string {
 }
 
 // NotFound satisfies interface github.com/docker/docker/errdefs.ErrNotFound
-func (n *notFoundError) NotFound() {}
+func (notFoundError) NotFound() {}