Removes unneeded Registered field

[ChrisJBurns]

Summary

The Registered field in Repository was not being used anywhere and given that Repository is only used to represent registered repositories, we should remove it.

Fixes #2628

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

From initial looks, the field was not being used therefore not influencing tests.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 53.137%. remained the same
when pulling 5ae6b4d on ChrisJBurns:main
into 75dcc45 on stacklok:main.

[coveralls]

coverage: 53.143% (+0.006%) from 53.137%
when pulling 5ae6b4d on ChrisJBurns:main
into 75dcc45 on stacklok:main.

[coveralls]

coverage: 53.137%. remained the same
when pulling c4c711e on ChrisJBurns:main
into 75dcc45 on stacklok:main.

[coveralls]

coverage: 53.137%. remained the same
when pulling 7513600 on ChrisJBurns:main
into 75dcc45 on stacklok:main.

[coveralls]

coverage: 53.137%. remained the same
when pulling ffcf69e on ChrisJBurns:main
into 75dcc45 on stacklok:main.

[coveralls]

coverage: 53.137%. remained the same
when pulling 39b4de1 on ChrisJBurns:main
into 75dcc45 on stacklok:main.

[evankanderson]

It looks like we may have originally intended to use Repository in ListRemoteRepositoriesFromProvider, but we changed that to use an UpstreamRepositoryRef, so this is, indeed, unused.