cli/trust: remove special handling for "plugin" Class

[thaJeztah]

• relates to vendor: update docker/docker to master (v24.0.0-dev) #4010
• relates to Move filtered registry search out of the image service moby/moby#45086 (moby/moby@7b3acdf removed the interface, so we can no longer use that).
• relates to HTTP GET a plugin manifest by version fails with 401 hub-feedback#2107
• replaces / closes Remove registry Service use #4111

This code depended on the registry Service interface, which has been removed,
so needed to be refactored. Digging further into the reason this code existed,
it looked like the Class=plugin was previously required on Docker Hub to handle
plugins, but this requirement is no longer there, so we can remove this special
handling.

This patch removes the special handling to both remove the use of the registry.Service
interface, as well as removing complexity that is no longer needed.

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

... and now I'm wondering if this Class is needed at all anymore (of there's only "image" or "").

I do see some handling in "distribution"; https://github.com/distribution/distribution/blob/e5d5810851d1f17a5070e9b6f940d8af98ea3c29/registry/client/auth/session.go#L154-L164

// String returns the string representation of the repository
// using the scope grammar
func (rs RepositoryScope) String() string {
	repoType := "repository"
	// Keep existing format for image class to maintain backwards compatibility
	// with authorization servers which do not support the expanded grammar.
	if rs.Class != "" && rs.Class != "image" {
		repoType = fmt.Sprintf("%s(%s)", repoType, rs.Class)
	}
	return fmt.Sprintf("%s:%s:%s", repoType, rs.Repository, strings.Join(rs.Actions, ","))
}

And that looks to be "only if it's NOT "" or "image"

[codecov-commenter]

Codecov Report

Merging #4114 (2dc9fb9) into master (bfe87fd) will increase coverage by 0.01%.
The diff coverage is 57.14%.

❗ Current head 2dc9fb9 differs from pull request most recent head 46095c6. Consider uploading reports for the commit 46095c6 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4114      +/-   ##
==========================================
+ Coverage   59.14%   59.16%   +0.01%     
==========================================
  Files         287      287              
  Lines       24741    24717      -24     
==========================================
- Hits        14634    14623      -11     
+ Misses       9221     9210      -11     
+ Partials      886      884       -2