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

[thaJeztah]

relates to:

• Include plugins in docker --badopt help output #1850
  • calling docker --badopt doesn't show cli plugins #1813
• cli: FlagErrorFunc: don't print long usage output for invalid flags #5233

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

- What I did

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/docker/docker.go	0.00%	2 Missing ⚠️

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[thaJeztah]

LOL; and all of a sudden, the linter starts to flag unrelated code;

70.72 cmd/docker/docker.go:560:74: hideUnsupportedFeatures - result 0 (error) is always nil (unparam)
70.72 func hideUnsupportedFeatures(cmd *cobra.Command, details versionDetails) error {
70.72 ^
70.72 1 issues: