Skip Redundant Persistent Ref existence check

What does this MR do?

This MR is to skip the redundant persistent ref existence check. This pressurizes Gitaly nodes and if timeout happens, it could end up fetching all refspecs in runner. The assumption was described by @stanhu at gitlab-com/gl-infra/production#2143 (comment 343690877)

Ok, maybe this is what happened based on https://log.gprd.gitlab.net/goto/15e8e5a9084502a3b17113fa0e0c1e60:

1. The ref did, in fact, exist. It seems like there are many chances to create the ref since every build attempts to do so.
2. However, the RefExists call timed out.
3. As a result, https://gitlab.com/gitlab-org/gitlab/blob/2bfe8f06724470576e408f3cc282a30f0c271880/app/models/ci/persistent_ref.rb#L18-19 returned false, and the CI pipeline got an empty refspec.

I still wonder why so many RPCs were stalling out, but perhaps we should cache the existence of this ref?

Screenshots

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
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