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 https://gitlab.com/gitlab-org/gitlab/issues/38535