cli/trust: GetNotaryRepository: remove uses of RepositoryInfo.Class

[thaJeztah]

• relates to cli/trust: remove special handling for "plugin" Class #4114
• relates to registry: deprecate RepositoryInfo.Class moby/moby#49006

The Class field was added because Docker Hub registry required a special scope to be set for pulling plugins;

HTTP/1.1 401 Unauthorized
...
Www-Authenticate: Bearer realm="https://auth.docker.io/token",service="registry.docker.io",scope="repository(plugin):vieux/sshfs:pull",error="insufficient_scope"

This is no longer a requirement, and the field is no longer set.

updates 0ba820e

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.53%. Comparing base (61b02e6) to head (ed9fcf3).
Report is 2 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #5660 +/- ##
=======================================
Coverage 59.53% 59.53% =======================================
Files 346 346 Lines 29357 29356 -1 =======================================
Hits 17477 17477 + Misses 10910 10909 -1 
Partials 970 970