Retire support for USGov and China cloud testing

Given the different Azure clouds are much more consistent now we no longer need to explicitly test in UsGov or China clouds. That will reduce our maintenance costs for those resources.

We should remove support for them from the following locations.

- [x] From resource [clean-up pipeline](https://github.com/Azure/azure-sdk-tools/blob/b0eb701900c22c4b60931f4a44ae2162c3e79d2a/eng/pipelines/live-test-cleanup.yml)
- [x] From all the [test/ci.yml files in all our repos](https://github.com/search?q=org%3AAzure+UsGov+language%3AYaml&type=code). We should keep support for other clouds in our common resource deployments and testing templates in case a partner team wants to run tests in their own tenant and sub in those clouds.
- [ ] We should remove our test subscriptions for those tenants.
- [ ] We should update our documentation to state anyone needing one-off testing for a custom repro should work with the service team partners to get access to the resources. We should also link to the sovereign cloud testing documentation for those that might need to do this manually.