[9.1] [Scout] De-duplicate Scout Cases API tests (#237144)

Scout API helper tests are currently in two places in the 9.1 version
branch:

* `src/platform/packages/shared/kbn-scout/integration_tests` ← ✅ all
Scout API tests are currently here, including the Cases API Scout tests
* `src/platform/packages/shared/kbn-scout/test/scout/tests/api_services`
← Just the Cases API Scout tests are here, making them a duplicate, so
**this PR deletes these duplicate files**

Author: csr