cmd/docker: setFlagErrorFunc: don't load plugins for invalid flags

On Docker CLI versions before v28.0.0, using an unknown flag would print
the usage output, showing all available top-level flags and commands;

    docker --badopt
    unknown flag: --badopt
    See 'docker --help'.

    Usage:  docker [OPTIONS] COMMAND

    A self-sufficient runtime for containers

    Options:
          --config string      Location of client config files (default "/root/.docker")
    ...

This output did not include plugin-commands, making the usage output
incomplete. That issue was fixed in [cli@40a6cf7], which loaded all
available cli-plugins, so that a stub was created for printing the
plugin commands in the usage output. Similarly, [cli@79a75da] added
code to hide experimental commands and commands not supported by the
daemon.

However, since 28.0.0 (commit [cli@f28fc7f]), the usage output was
removed for this error, so loading plugins is no longer needed;

    docker --badopt
    unknown flag: --badopt

    Usage:  docker [OPTIONS] COMMAND [ARG...]

    Run 'docker --help' for more information

This patch removes the code added in [cli@40a6cf7] and [cli@79a75da].

With this patch, the output is still the same;

    docker --unknown-flag buildx ls --no-such
    unknown flag: --unknown-flag

    Usage:  docker [OPTIONS] COMMAND [ARG...]

    Run 'docker --help' for more information

This function only handles flags defined by the CLI itself; invalid
flags for plugins are handled by the plugin itself, so are not
impacted;

    docker buildx ls --no-such
    unknown flag: --no-such

    Usage:  docker buildx ls

    Run 'docker buildx ls --help' for more information

[cli@f28fc7f]: https://github.com/docker/cli/commit/f28fc7f82fc87d0ed521de452b6227cee76fd956
[cli@40a6cf7]: https://github.com/docker/cli/commit/40a6cf7c477cf328134b0b8dcdbd9a09d02f918b
[cli@79a75da]: https://github.com/docker/cli/commit/79a75da0fd97b02311676028c3406b242c785f7c

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

cmd/docker/docker.go

@@ -181,15 +181,9 @@ func setFlagErrorFunc(dockerCli command.Cli, cmd *cobra.Command) {
 	// is called.
 	flagErrorFunc := cmd.FlagErrorFunc()
 	cmd.SetFlagErrorFunc(func(cmd *cobra.Command, err error) error {
-		if err := pluginmanager.AddPluginCommandStubs(dockerCli, cmd.Root()); err != nil {
-			return err
-		}
 		if err := isSupported(cmd, dockerCli); err != nil {
 			return err
 		}
-		if err := hideUnsupportedFeatures(cmd, dockerCli); err != nil {
-			return err
-		}
 		return flagErrorFunc(cmd, err)
 	})
 }