Fix flaky epic deletion specs

What does this MR do and why?

Since we are deleting the objects within the spec, we need to refind the object so the model is properly marked as persisted before deletion.

References

• #537934 (closed)
• #537935 (closed)
• #537936 (closed)
• #537937 (closed)
• gitlab-org/quality/engineering-productivity/master-broken-incidents#14684 (closed)

How to set up and validate locally

You can reproduce the failure with:

bin/rspec './ee/spec/services/ee/issuable/destroy_service_spec.rb' --seed 40885

MR acceptance checklist

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