Remove double locking as the underlying codebase has changed

Summary

Currently, inside app/services/ci/retry_job_service.rb, the clone function skips the transaction check

Gitlab::ExclusiveLease.skipping_transaction_check

We want to get rid of this workaround by removing double locking.

For further information, take a look at: !166146 (closed)

Improvements

Removes the double locking for the RetryJobService.

Risks

Since we don't know, if there might arise complications for the production environment, we hid this refactoring behind a feature flag, that is turned off as a default value.

Involved components

Optional: Intended side effects

Optional: Missing test coverage