[api-go] Remove cloud submodule and cloud protos

**Describe the solution you'd like**

Now that https://github.com/temporalio/cloud-sdk-go exists and now that #1831 has been out for a couple of releases now, we should remove the cloud submodule and cloud protos from https://github.com/temporalio/api-go, mentioning in the PR that https://github.com/temporalio/cloud-sdk-go is the home for them now.