Fix order-dependent spec in Gitaly cleanup spec

Stubs for Gitaly calls are memoized by storage and service names in the @stubs class variable. Normally this doesn't cause a problem, but an example in spec/lib/gitlab/gitaly_client/cleanup_service_spec.rb expected that a new stub would always be instantiated. Since spec/services/projects/cleanup_service_spec.rb preceded spec/lib/gitlab/gitaly_client/cleanup_service_spec.rb, the stub had already been instantiated, causing the test to fail.

To fix this problem, we just clear out the Gitaly stubs for spec/lib/gitlab/gitaly_client/cleanup_service_spec.rb.

Closes #38535 (closed)