Do not fail when cleaning up MR with no repository

What does this MR do?

When we clean up refs of a merge request that is on a project with no repository anymore, the MergeRequests::CleanupRefsService will fail.

Before this fix, it'll throw an error and retry and will not set the cleanup schedule as completed. When the cleanup schedule is not marked as completed, it'll be picked up by the scheduler again.

To fix that, we are now checking if the repository exists before deleting refs. This way, we can still update the cleanup schedule.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #245263 (closed)