db: remove deprecated Repos.Delete

Created by: keegancsmith

Repositories are managed by repo-updater. To quote the documentation of the graphql mutation:

DEPRECATED: All repositories are accessible or deleted. To prevent a repository from being accessed on Sourcegraph add it to the external service exclude configuration. It will be deleted. This mutation will be removed in 3.6.

Deletes a repository and all data associated with it, irreversibly.

If the repository was added because it was present in the site configuration (directly, or because it originated from a configured code host), then it will be re-added during the next sync. If you intend to make the repository inaccessible to users and not searchable, use setRepositoryEnabled to disable the repository instead of deleteRepository.